Ratio and proportion reasoning
By the end of this chapter you will be able to
- Simplify a ratio and find a missing amount from a ratio
- Share an amount in a given ratio and write each share
- Use the unit method to scale a recipe or a rate
- Compare the price of one item to decide which is better value
- Find a scale factor and use it to find a missing length
You should already know
- Multiplication and division facts, and equivalent fractions (Year 4 and Year 5)
- Fractions of amounts and percentages (Chapter 2 of this book)
Where this leads
- Measures in context (Chapter 4 of this book)
KS2 SATs practice for reasoning (Year 6). Original questions in the style of the national tests. Content: Solve problems involving the relative sizes of two quantities where missing values can be found by integer multiplication or division (Year 5-6), Use the ratio language a:b and the link between ratio and fractions (Year 6), Solve problems of unequal sharing and grouping using fractions and multiples (Year 6), Use the unit method for proportion, including percentages of a whole (Year 5-6), Solve problems involving similar shapes where the scale factor is known or can be found (Year 6), Use proportional reasoning to compare prices and quantities, and explain why a comparison is or is not fair (Year 5-6).
Exercise P Do you remember?
Non-calculatorMethod. Divide both parts by 2.
- Find the common factor. 2 divides 4 and 6.2
- Divide. 4 / 2 = 2 and 6 / 2 = 3.2:3
- Write the answer. 2:3
Check. Both parts of 2:3 multiplied by 2 give 4:6. ✓
Method. Divide by 4.
- Divide by 4. 28 / 4 = 7.7
- Write the answer. 7
Check. 7 x 4 = 28. ✓
Method. Divide by 10.
- Find 10%. 250 / 10 = 25.25
- Write the answer. 25
Check. 25 x 10 = 250. ✓
Method. Split 15 into 10 and 5.
- Multiply by 10. 6 x 10 = 60.60
- Multiply by 5. 6 x 5 = 30.30
- Add. 60 + 30 = 90.90
- Write the answer. 90
Check. 90 / 6 = 15. ✓
Method. Divide the new length by the old.
- Divide. 12 / 3 = 4.4
- Write the answer. 4
Check. 3 x 4 = 12. ✓
Ratio language and simplifying ratios
Rule
A ratio compares two amounts. 3:5 means for every 3 of one thing there are 5 of the other.
Write a ratio in its simplest form by dividing both parts by the same number. 12:18 = 2:3.
To find a missing amount, find the value of one part first, then multiply.
Key ideas
- Divide both parts of a ratio by the same number to simplify it.
- In a ratio of 2:3 there are 5 parts altogether, so the first part is 2/5 of the total.
- Find the value of one part, then multiply, to find a missing amount.
Method. Divide both parts of the ratio by the highest common factor.
- Find the highest common factor of both numbers. The biggest number that divides 15 and 25 is 5.HCF = 5
- Divide both parts by it. 15 / 5 = 3 and 25 / 5 = 5.3:5
- Write the answer. 3:5
Check. 3 and 5 have no common factor except 1. ✓
Watch out. Both parts must be divided by the same number, just as with a fraction.
Method. Find the value of one part, then multiply.
- Find the value of one part. 18 is 3 parts, so one part is 18 / 3.1 part = 6
- Multiply to find the other amount. There are 5 parts of blue.5 x 6 = 30
- Write the answer. 30
Check. 30 : 18 = 3:5. ✓
Watch out. Do not add the ratio numbers to the amount. Find one part first.
Method. Find the value of one part, then multiply.
- Find the value of one part. 21 is 7 parts, so one part is 21 / 7.1 part = 3
- Multiply to find the other amount. There are 2 parts of cats.2 x 3 = 6
- Write the answer. 6
Check. 6 : 21 = 2:7. ✓
Watch out. Do not add the ratio numbers to the amount. Find one part first.
Watch out
Watch out. A ratio is not a fraction of the whole. In 2:3 there are 5 parts altogether, so the first part is 2/5 of the total.
Exam tip
Practice guide. Write the ratio above the amounts in a table, then fill in the missing value.
Exercise 3A Practice: ratio language and simplifying
Non-calculatorMethod. Divide both parts of the ratio by the highest common factor.
- Find the highest common factor of both numbers. The biggest number that divides 12 and 20 is 4.HCF = 4
- Divide both parts by it. 12 / 4 = 3 and 20 / 4 = 5.3:5
- Write the answer. 3:5
Check. 3 and 5 have no common factor except 1. ✓
Watch out. Both parts must be divided by the same number, just as with a fraction.
Method. Divide both parts of the ratio by the highest common factor.
- Find the highest common factor of both numbers. The biggest number that divides 45 and 30 is 15.HCF = 15
- Divide both parts by it. 45 / 15 = 3 and 30 / 15 = 2.3:2
- Write the answer. 3:2
Check. 3 and 2 have no common factor except 1. ✓
Watch out. Both parts must be divided by the same number, just as with a fraction.
Method. Find the value of one part, then multiply.
- Find the value of one part. 12 is 2 parts, so one part is 12 / 2.1 part = 6
- Multiply to find the other amount. There are 3 parts of girls.3 x 6 = 18
- Write the answer. 18
Check. 18 : 12 = 2:3. ✓
Watch out. Do not add the ratio numbers to the amount. Find one part first.
Method. Find the value of one part, then multiply.
- Find the value of one part. 35 is 5 parts, so one part is 35 / 5.1 part = 7
- Multiply to find the other amount. There are 4 parts of apples.4 x 7 = 28
- Write the answer. 28
Check. 28 : 35 = 4:5. ✓
Watch out. Do not add the ratio numbers to the amount. Find one part first.
Method. Find the value of one part, then multiply.
- Find the value of one part. 49 is 7 parts, so one part is 49 / 7.1 part = 7
- Multiply to find the other amount. There are 3 parts of blue.3 x 7 = 21
- Write the answer. 21
Check. 21 : 49 = 3:7. ✓
Watch out. Do not add the ratio numbers to the amount. Find one part first.
Method. Find the value of one part, then multiply.
- Find the value of one part. 25 is 5 parts, so one part is 25 / 5.1 part = 5
- Multiply to find the other amount. There are 2 parts of laps.2 x 5 = 10
- Write the answer. 10
Check. 10 : 25 = 5:2. ✓
Watch out. Do not add the ratio numbers to the amount. Find one part first.
Method. Test each choice against the question.
- Read what the question asks. Decide what the answer must do or show.
- Test the choice. The ratio 2:5 means 2 cats for every 5 dogs. The other two choices reverse or add the parts.For every 2 cats there are 5 dogs fits
- Reject the other choices. The ratio 2:5 has 7 parts in total, but this does not mean there are 7 cats for every 2 dogs.For every 5 cats there are 2 dogs and For every 7 cats there are 2 dogs do not fit
- Write the answer. For every 2 cats there are 5 dogs
Check. For every 2 cats there are 5 dogs is the one that fits. ✓
Method. Test each option.
- Check each option. Test each one against the rule or the calculation.
- Tick the true ones. Each ticked ratio can be divided or multiplied to give 2:3: 4:6 is 2:3 x 2, and 6:9 is 2:3 x 3.4:6, 6:9
- Leave the others unticked. These do not match.3:2, 5:6, 8:10 are not ticked
- Write the answer. 4:6, 6:9
Check. Each ticked option is true. ✓
Watch out. 3:2 is the same numbers in the other order, so it is not equivalent.
Method. Find the value of one part, then multiply.
- Find the value of one part. 240 is 4 parts, so one part is 240 / 4.1 part = 60
- Multiply to find the other amount. There are 1 parts of sugar.1 x 60 = 60
- Write the answer. 60
Check. 60 : 240 = 1:4. ✓
Watch out. Do not add the ratio numbers to the amount. Find one part first.
Method. Find the total number of parts, then the value of one part.
- Find the total parts. 3 + 4 = 7 parts.7 parts
- Find one part. 28 / 7 = 4 pupils.4
- Find the girls. 3 parts of girls: 3 x 4 = 12.12
- Write the answer. 12
Check. 12 girls and 16 boys make 28. ✓
Method. Divide both parts by the same number.
- Divide both parts by 3. 3 / 3 = 1 and 6 / 3 = 2.1:2
- Compare. So 3:6 = 1:2.3:6 = 1:2
- Write the answer. Yes, because 3:6 divided by 3 on both parts is 1:2.
Check. 3:6 and 1:2 are equivalent. ✓
Method. Find the total number of parts, then the value of one part.
- Total parts. 5 + 3 = 8 parts.8 parts
- One part. 40 / 8 = 5 bags.5 bags
- Each flavour. Salt is 5 x 5 = 25 and vinegar is 3 x 5 = 15.25 and 15
- Write the answer. 25, 15
Check. 25 + 15 = 40. ✓
Method. Work out the total number of parts first.
- Total parts. 2 + 5 = 7 parts.7 parts
- Fraction for the first part. The first part is 2 of the 7 parts, so 2/7.2/7
- Write the answer. No, because the first part is 2/7 of the total, since 2 + 5 = 7 is the total number of parts.
Check. 2/7 + 5/7 = 1. ✓
Method. Find the value of one part from the blue beads, then multiply by 4.
- One part. 36 / 9 = 4 beads.4
- Red beads. 4 parts of red: 4 x 4 = 16.16
- Write the answer. 16
Check. 16 : 36 = 4 : 9. ✓
Method. Check whether 6:9 and 2:3 are equivalent.
- Simplify 6:9. Divide both by 3: 6 / 3 = 2 and 9 / 3 = 3.2:3
- Compare. The simplified ratio is the same, so the ratios are equivalent.yes
- Write the answer. yes
Check. 6:9 = 2:3. ✓
Watch out. Equivalent ratios give the same relationship, but the actual amounts are different.
Method. Find the value of one part.
- Total parts. 3 + 2 = 5 parts.5 parts
- One part. 20 / 5 = 4.4
- Blue. 2 parts of blue: 2 x 4 = 8.8
- Write the answer. 8
Check. 12 red and 8 blue make 20. ✓
Sharing in a ratio and fractions of a total
Rule
To share in a ratio, add the parts to find the total number of parts. Then find the value of one part.
A part of a ratio is a fraction of the total. In 2:3, the first part is 2/5 of the total, because 2 + 3 = 5.
Key ideas
- Total parts first: 1 + 3 = 4 parts.
- The value of one part = total amount / total parts.
- Each person gets their ratio number of parts.
Method. Find the total parts, then the value of one part.
- Find the total number of parts. 1 + 3 = 4 parts.4 parts
- Find the value of one part. 120 / 4 = 30.1 part = 30
- Multiply each ratio number by one part. Ali: 1 x 30 and Bea: 3 x 3030, 90
- Write the answer. 30, 90
Check. 30 + 90 = 120. ✓
Watch out. Check that the amounts add up to the total.
Method. The total is the sum of all the parts, so the fraction is that part over the total.
- Find the total number of parts. 2 + 3 = 5.5 parts
- Write the part as a fraction of the total, in simplest form. The part is 2 out of 5 parts, which simplifies to 2/5.2/5
- Write the answer. 2/5
Check. 2 + 3 = 5. ✓
Watch out. The fraction uses the total of all parts, not the other part only.
Method. Find the total parts, then the value of one part.
- Find the total number of parts. 2 + 3 + 5 = 10 parts.10 parts
- Find the value of one part. 300 / 10 = 30.1 part = 30
- Multiply each ratio number by one part. Ann: 2 x 30 and Ben: 3 x 30 and Cal: 5 x 3060, 90, 150
- Write the answer. 60, 90, 150
Check. 60 + 90 + 150 = 300. ✓
Watch out. Check that the amounts add up to the total.
Watch out
Watch out. Share the money into the number of parts, not into the number of people.
Exam tip
Practice guide. Check that your shares add up to the total before you write the answer.
Exercise 3B Practice: sharing in a ratio
Non-calculatorMethod. Find the total parts, then the value of one part.
- Find the total number of parts. 1 + 2 = 3 parts.3 parts
- Find the value of one part. 90 / 3 = 30.1 part = 30
- Multiply each ratio number by one part. Tom: 1 x 30 and Sam: 2 x 3030, 60
- Write the answer. 30, 60
Check. 30 + 60 = 90. ✓
Watch out. Check that the amounts add up to the total.
Method. Find the total parts, then the value of one part.
- Find the total number of parts. 3 + 5 = 8 parts.8 parts
- Find the value of one part. 144 / 8 = 18.1 part = 18
- Multiply each ratio number by one part. Mia: 3 x 18 and Leo: 5 x 1854, 90
- Write the answer. 54, 90
Check. 54 + 90 = 144. ✓
Watch out. Check that the amounts add up to the total.
Method. Find the total parts, then the value of one part.
- Find the total number of parts. 2 + 3 + 5 = 10 parts.10 parts
- Find the value of one part. 500 / 10 = 50.1 part = 50
- Multiply each ratio number by one part. Ava: 2 x 50 and Ben: 3 x 50 and Cy: 5 x 50100, 150, 250
- Write the answer. 100, 150, 250
Check. 100 + 150 + 250 = 500. ✓
Watch out. Check that the amounts add up to the total.
Method. The total is the sum of all the parts, so the fraction is that part over the total.
- Find the total number of parts. 3 + 4 = 7.7 parts
- Write the part as a fraction of the total, in simplest form. The part is 4 out of 7 parts, which simplifies to 4/7.4/7
- Write the answer. 4/7
Check. 4 + 3 = 7. ✓
Watch out. The fraction uses the total of all parts, not the other part only.
Method. The total is the sum of all the parts, so the fraction is that part over the total.
- Find the total number of parts. 1 + 3 = 4.4 parts
- Write the part as a fraction of the total, in simplest form. The part is 1 out of 4 parts, which simplifies to 1/4.1/4
- Write the answer. 1/4
Check. 1 + 3 = 4. ✓
Watch out. The fraction uses the total of all parts, not the other part only.
Method. The total is the sum of all the parts, so the fraction is that part over the total.
- Find the total number of parts. 2 + 5 + 3 = 10.10 parts
- Write the part as a fraction of the total, in simplest form. The part is 3 out of 10 parts, which simplifies to 3/10.3/10
- Write the answer. 3/10
Check. 3 + 7 = 10. ✓
Watch out. The fraction uses the total of all parts, not the other part only.
Method. The total is the sum of all the parts, so the fraction is that part over the total.
- Find the total number of parts. 2 + 4 = 6.6 parts
- Write the part as a fraction of the total, in simplest form. The part is 2 out of 6 parts, which simplifies to 1/3.1/3
- Write the answer. 1/3
Check. 2 + 4 = 6. ✓
Watch out. The fraction uses the total of all parts, not the other part only.
Method. Find the value of one part, then the difference in parts.
- Total parts. 2 + 3 = 5 parts.5 parts
- One part. 75 / 5 = 15 pounds.15 pounds
- Difference. Jo has 3 parts and Priya has 2 parts, so the difference is 1 part, which is 15 pounds.15 pounds
- Write the answer. 15
Check. Priya 30 and Jo 45: 45 - 30 = 15. ✓
Method. Divide both parts of the ratio by the highest common factor.
- Find the highest common factor of both numbers. The biggest number that divides 12 and 18 is 6.HCF = 6
- Divide both parts by it. 12 / 6 = 2 and 18 / 6 = 3.2:3
- Write the answer. 2:3
Check. 2 and 3 have no common factor except 1. ✓
Watch out. Both parts must be divided by the same number, just as with a fraction.
Method. Test each choice against the question.
- Read what the question asks. Decide what the answer must do or show.
- Test the choice. The total is 2 + 3 = 5 parts, so the first part is 2/5.No, it is 2/5 of the total fits
- Reject the other choices. 2/3 compares the first part with the second, not with the total.Yes, it is 2/3 of the total and No, it is 2/3 of the second part only do not fit
- Write the answer. No, it is 2/5 of the total
Check. No, it is 2/5 of the total is the one that fits. ✓
Method. Find the value of one part.
- Total parts. 2 + 1 = 3 parts.3 parts
- One part. 30 / 3 = 10.10
- Sopranos. 2 parts: 2 x 10 = 20.20
- Write the answer. 20
Check. 20 sopranos and 10 altos make 30. ✓
Method. Think about the parts.
- Look at the parts. The first amount has 1 part and the second has 2 parts.1 and 2
- Compare. 2 parts is twice 1 part.twice as big
- Write the answer. Because for each one part of the first amount there are two parts of the second, so the second is twice as big.
Check. 2 = 2 x 1. ✓
Method. Find the value of one part from Tom's share.
- One part. 24 / 4 = 6 sweets.6
- Total. There are 4 + 5 = 9 parts, so 9 x 6 = 54.54
- Write the answer. 54
Check. 24 + 30 = 54. ✓
Method. Find the value of one part from the largest share.
- One part. 36 / 3 = 12 pounds.12
- Total parts. 1 + 2 + 3 = 6 parts, so 6 x 12 = 72.72
- Write the answer. 72
Check. 12 + 24 + 36 = 72. ✓
Method. Find the total number of parts, then write the middle part as a fraction.
- Total parts. 1 + 2 + 3 = 6 parts.6 parts
- Middle part. The middle person has 2 parts, so 2/6.2/6
- Simplify. The HCF of 2 and 6 is 2, so 2/6 = 1/3.1/3
- Write the answer. 1/3
Check. 2/6 = 1/3. ✓
The unit method and percentages
Rule
The unit method means finding the amount for ONE, then multiplying by the number you need.
For a percentage, find 1% of the whole first, then multiply by the percentage you want. If 75% costs 60 pounds, then 1% is 60 / 75 pounds, and 100% is 80 pounds.
Key ideas
- Find one first, then multiply.
- A percentage of a whole: find 1% first.
- When a percentage is taken off, the price paid is the percentage that is left.
Method. Find the amount for one person, then multiply.
- Find the amount for one. 300 g is for 4, so one is 300 / 4.1 = 75 g
- Multiply for the new number. Multiply by 7.75 x 7 = 525
- Write the answer. 525 g
Check. 525 / 7 = 75, the same amount for each person. ✓
Watch out. Doubling the people does not always need double the ingredients unless you check the unit.
Method. Find 1% of the original price, then multiply by 100.
- Work out 1 per cent. 60 pounds is 75% of the price, so 1% is 60 / 75.1% = 4/5
- Work out 100 per cent. The original price is 100%, so multiply 1% by 100.4/5 x 100 = 80
- Write the answer. 80
Check. 80 less 25% is 60. ✓
Watch out. The discount is taken from the original price, so the price paid is what is left.
Method. Find the amount for one person, then multiply.
- Find the amount for one. 250 g is for 5, so one is 250 / 5.1 = 50 g
- Multiply for the new number. Multiply by 8.50 x 8 = 400
- Write the answer. 400 g
Check. 400 / 8 = 50, the same amount for each person. ✓
Watch out. Doubling the people does not always need double the ingredients unless you check the unit.
Watch out
Watch out. When a shop takes 20% off, the price paid is 80% of the original, not 20%.
Exam tip
Practice guide. Write the amount for one at the top of a small table, then multiply down.
Exercise 3C Practice: the unit method and percentages
Non-calculatorMethod. Find the amount for one person, then multiply.
- Find the amount for one. 240 g is for 6, so one is 240 / 6.1 = 40 g
- Multiply for the new number. Multiply by 9.40 x 9 = 360
- Write the answer. 360 g
Check. 360 / 9 = 40, the same amount for each person. ✓
Watch out. Doubling the people does not always need double the ingredients unless you check the unit.
Method. Find the amount for one person, then multiply.
- Find the amount for one. 45 ml is for 3, so one is 45 / 3.1 = 15 ml
- Multiply for the new number. Multiply by 8.15 x 8 = 120
- Write the answer. 120 ml
Check. 120 / 8 = 15, the same amount for each person. ✓
Watch out. Doubling the people does not always need double the ingredients unless you check the unit.
Method. Find the amount for one person, then multiply.
- Find the amount for one. 1200 ml is for 4, so one is 1200 / 4.1 = 300 ml
- Multiply for the new number. Multiply by 6.300 x 6 = 1800
- Write the answer. 1800 ml
Check. 1800 / 6 = 300, the same amount for each person. ✓
Watch out. Doubling the people does not always need double the ingredients unless you check the unit.
Method. Find the amount for one person, then multiply.
- Find the amount for one. 350 g is for 5, so one is 350 / 5.1 = 70 g
- Multiply for the new number. Multiply by 12.70 x 12 = 840
- Write the answer. 840 g
Check. 840 / 12 = 70, the same amount for each person. ✓
Watch out. Doubling the people does not always need double the ingredients unless you check the unit.
Method. Find 1% of the original price, then multiply by 100.
- Work out 1 per cent. 72 pounds is 80% of the price, so 1% is 72 / 80.1% = 9/10
- Work out 100 per cent. The original price is 100%, so multiply 1% by 100.9/10 x 100 = 90
- Write the answer. 90
Check. 90 less 20% is 72. ✓
Watch out. The discount is taken from the original price, so the price paid is what is left.
Method. Find 1% of the original price, then multiply by 100.
- Work out 1 per cent. 45 pounds is 60% of the price, so 1% is 45 / 60.1% = 3/4
- Work out 100 per cent. The original price is 100%, so multiply 1% by 100.3/4 x 100 = 75
- Write the answer. 75
Check. 75 less 40% is 45. ✓
Watch out. The discount is taken from the original price, so the price paid is what is left.
Method. Find the amount for one person, then multiply.
- Find the amount for one. 200 g is for 8, so one is 200 / 8.1 = 25 g
- Multiply for the new number. Multiply by 12.25 x 12 = 300
- Write the answer. 300 g
Check. 300 / 12 = 25, the same amount for each person. ✓
Watch out. Doubling the people does not always need double the ingredients unless you check the unit.
Method. Find the cost of one mile, then multiply by 7.
- One mile. 6 / 2 = 3 pounds.3 pounds
- Seven miles. 3 x 7 = 21 pounds.21 pounds
- Write the answer. 21
Check. 21 / 7 = 3. ✓
Method. Find the distance in one minute, then multiply by 40. Or work out how many 15-minute sets there are.
- Find the number of 15-minute sets. 40 is not a whole number of 15s, so find 5 minutes first.15 minutes = 3 km, so 5 minutes = 1 km
- Multiply for 40 minutes. 40 minutes = 8 lots of 5 minutes, so 8 km.8 x 1 km = 8 km
- Write the answer. 8
Check. 8 km in 40 minutes, because 15 minutes is 3 km and 5 minutes is 1 km. ✓
Method. Compare the number of people with the number of pizzas.
- Find the pizzas for one person. 2 pizzas for 4 people is 2/4 = 1/2 pizza each.1/2 pizza each
- Check for 8 people. 8 x 1/2 = 4 pizzas.4 pizzas
- Write the answer. Yes, because 8 people is double 4 people, so they need double the pizzas: 2 x 2 = 4.
Check. 8 x 1/2 = 4. ✓
Method. Find 1% of the original price, then multiply by 100.
- Work out 1 per cent. 48 pounds is 80% of the price, so 1% is 48 / 80.1% = 3/5
- Work out 100 per cent. The original price is 100%, so multiply 1% by 100.3/5 x 100 = 60
- Write the answer. 60
Check. 60 less 20% is 48. ✓
Watch out. The discount is taken from the original price, so the price paid is what is left.
Method. Find 1% of the original price, then multiply by 100.
- Work out 1 per cent. 36 pounds is 80% of the price, so 1% is 36 / 80.1% = 9/20
- Work out 100 per cent. The original price is 100%, so multiply 1% by 100.9/20 x 100 = 45
- Write the answer. 45
Check. 45 less 20% is 36. ✓
Watch out. The discount is taken from the original price, so the price paid is what is left.
Method. Find the cost for one, then use it.
- Cost for one litre. 2 pounds.2 pounds
- Cost for half a litre. 2 / 2 = 1 pound.1 pound
- Write the answer. Yes, because it finds the cost for one litre (2 pounds), then uses it to find the cost for half a litre (1 pound).
Check. 2 / 2 = 1. ✓
Method. Find the flour for one cake, then multiply by 4.
- One cake. 450 / 6 = 75 g.75 g
- Four cakes. 75 x 4 = 300 g.300 g
- Write the answer. 300
Check. 300 / 4 = 75. ✓
Best value and fair comparisons
Rule
To compare prices, find the price of one item. The lower price for one item is the better value.
To compare results fairly, use the same basis. Compare percentages when the totals are different.
Key ideas
- Price per item = total price / number of items.
- A bigger pack is not always cheaper per item.
- Compare percentages, not raw scores, when the number of tries is different.
Method. Work out the price of one item for each pack, then compare.
- Work out the price of one item in Pack A. 240p / 6 = 40p.40p
- Work out the price of one item in Pack B. 180p / 4 = 45p.45p
- Compare the prices of one item. The cheaper one for one item is the better value.Pack A is cheaper for one item
- Write the answer. Pack A
Check. Pack A costs less for each item. ✓
Watch out. Compare the price of ONE item. A bigger pack is not always cheaper for each item.
Method. Write each result as a percentage, then compare.
- Work out the percentage for Team A. 24 out of 30 is 24 / 30 x 100.80%
- Work out the percentage for Team B. 30 out of 40 is 30 / 40 x 100.75%
- Compare the two percentages. The higher percentage is the better result.Team A is higher
- Write the answer. Team A
Check. 80% and 75% are compared fairly. ✓
Watch out. Comparing 30 with 24 is not fair when the numbers of shots are different.
Method. Work out the price of one item for each pack, then compare.
- Work out the price of one item in Box X. 90p / 3 = 30p.30p
- Work out the price of one item in Box Y. 175p / 5 = 35p.35p
- Compare the prices of one item. The cheaper one for one item is the better value.Box X is cheaper for one item
- Write the answer. Box X
Check. Box X costs less for each item. ✓
Watch out. Compare the price of ONE item. A bigger pack is not always cheaper for each item.
Watch out
Watch out. Team B scored more goals in total, but they are not the better team if they had more attempts.
Exam tip
Practice guide. Put the same unit (pence per item, or percentage) on both sides before you compare.
Exercise 3D Practice: best value and fair comparisons
Non-calculatorMethod. Work out the price of one item for each pack, then compare.
- Work out the price of one item in Box X. 300p / 4 = 75p.75p
- Work out the price of one item in Box Y. 390p / 6 = 65p.65p
- Compare the prices of one item. The cheaper one for one item is the better value.Box Y is cheaper for one item
- Write the answer. Box Y
Check. Box Y costs less for each item. ✓
Watch out. Compare the price of ONE item. A bigger pack is not always cheaper for each item.
Method. Work out the price of one item for each pack, then compare.
- Work out the price of one item in Packet A. 400p / 5 = 80p.80p
- Work out the price of one item in Packet B. 600p / 8 = 75p.75p
- Compare the prices of one item. The cheaper one for one item is the better value.Packet B is cheaper for one item
- Write the answer. Packet B
Check. Packet B costs less for each item. ✓
Watch out. Compare the price of ONE item. A bigger pack is not always cheaper for each item.
Method. Write each result as a percentage, then compare.
- Work out the percentage for Sam. 18 out of 20 is 18 / 20 x 100.90%
- Work out the percentage for Lou. 21 out of 30 is 21 / 30 x 100.70%
- Compare the two percentages. The higher percentage is the better result.Sam is higher
- Write the answer. Sam
Check. 90% and 70% are compared fairly. ✓
Watch out. Comparing 30 with 24 is not fair when the numbers of shots are different.
Method. Write each result as a percentage, then compare.
- Work out the percentage for Team Red. 12 out of 15 is 12 / 15 x 100.80%
- Work out the percentage for Team Blue. 16 out of 25 is 16 / 25 x 100.64%
- Compare the two percentages. The higher percentage is the better result.Team Red is higher
- Write the answer. Team Red
Check. 80% and 64% are compared fairly. ✓
Watch out. Comparing 30 with 24 is not fair when the numbers of shots are different.
Method. Find the price of 1 g for each, or compare the prices of 100 g.
- Price of 100 g for 500 g. 250 / 5 = 50 pence per 100 g.50 pence
- Price of 100 g for 750 g. 360 / 7.5 = 48 pence per 100 g.48 pence
- Compare. 48 pence is less than 50 pence, so 750 g is better value.750 g
- Write the answer. 750 g
Check. 48 < 50. ✓
Method. Find the price of one litre for each jar.
- Jar 1. 2.10 / 1.5 = 1.40 pounds per litre.1.40 pounds
- Jar 2. 2.60 / 2 = 1.30 pounds per litre.1.30 pounds
- Compare. 1.30 is less than 1.40, so 2 litres is cheaper.2 litres
- Write the answer. 2 litres
Check. 1.30 < 1.40. ✓
Method. Test each choice against the question.
- Read what the question asks. Decide what the answer must do or show.
- Test the choice. Percentages put both shops on the same basis of 100 customers.Compare the percentage of customers who bought something fits
- Reject the other choices. The number of sales depends on how many customers each shop had.Compare only the number of sales and Compare the number of shops do not fit
- Write the answer. Compare the percentage of customers who bought something
Check. Compare the percentage of customers who bought something is the one that fits. ✓
Method. Work out the price for one item.
- Find one price. Divide the total price by the number of items.price / items
- Compare. The smaller price for one item is the better value.lower price per item
- Write the answer. Because the price per item may be higher in the bigger pack, so you must divide the price by the number of items.
Check. Price per item decides the value. ✓
Method. Write each result as a percentage, then compare.
- Work out the percentage for Ravi. 15 out of 20 is 15 / 20 x 100.75%
- Work out the percentage for Ben. 36 out of 50 is 36 / 50 x 100.72%
- Compare the two percentages. The higher percentage is the better result.Ravi is higher
- Write the answer. Ravi
Check. 75% and 72% are compared fairly. ✓
Watch out. Comparing 30 with 24 is not fair when the numbers of shots are different.
Method. Compare the number of tickets as a percentage of the seats.
- Week 1. 180 / 300 = 60%.60%
- Week 2. 300 / 300 = 100%.100%
- Compare. 100% is more than 60%, so week 2 was better.Week 2
- Write the answer. Week 2
Check. 100% > 60%. ✓
Method. Write both as percentages.
- Write as percentages. 9 out of 10 is 90%, and 19 out of 20 is 95%.90% and 95%
- Compare. 95% is more than 90%.19 out of 20 is better
- Write the answer. Because 9 out of 10 is 90%, and 19 out of 20 is 95%, so 19 out of 20 is better.
Check. 95 > 90. ✓
Similar shapes and scale factors
Rule
Two shapes are similar when one is an enlargement of the other. All lengths are multiplied by the same scale factor.
Scale factor = new length / original length. Missing length = original length x scale factor.
Key ideas
- Scale factor = new length / original length.
- Every length is multiplied by the same scale factor.
- Angles stay the same in similar shapes.
Method. Divide the new length by the original length.
- Write the scale factor as a division. The scale factor is the new length divided by the original.12 / 4
- Work it out. 12 / 4 = 3.3
- Write the answer. 3
Check. 4 x 3 = 12. ✓
Watch out. A scale factor bigger than 1 makes a shape bigger.
Method. Multiply each length of the original by the scale factor.
- Use the scale factor. Each length is multiplied by the scale factor 2.5.6 x 2.5
- Work it out. 6 x 2.5 = 15.15
- Write the answer. 15
Check. 6 x 2.5 = 15, and 8 x 2.5 = 20. ✓
Watch out. Check the scale factor with the side you know first.
Method. Divide the new length by the original length.
- Write the scale factor as a division. The scale factor is the new length divided by the original.20 / 5
- Work it out. 20 / 5 = 4.4
- Write the answer. 4
Check. 5 x 4 = 20. ✓
Watch out. A scale factor bigger than 1 makes a shape bigger.
Watch out
Watch out. Find the scale factor from one pair of matching sides, then use it for every other length.
Exam tip
Practice guide. Write the scale factor next to the shape, so you can use it again.
Exercise 3E Practice: similar shapes and scale factors
Non-calculatorMethod. Divide the new length by the original length.
- Write the scale factor as a division. The scale factor is the new length divided by the original.10 / 4
- Work it out. 10 / 4 = 2.5.2.5
- Write the answer. 2.5
Check. 4 x 2.5 = 10. ✓
Watch out. A scale factor bigger than 1 makes a shape bigger.
Method. Divide the new length by the original length.
- Write the scale factor as a division. The scale factor is the new length divided by the original.21 / 7
- Work it out. 21 / 7 = 3.3
- Write the answer. 3
Check. 7 x 3 = 21. ✓
Watch out. A scale factor bigger than 1 makes a shape bigger.
Method. Multiply each length of the original by the scale factor.
- Use the scale factor. Each length is multiplied by the scale factor 3.5 x 3
- Work it out. 5 x 3 = 15.15
- Write the answer. 15
Check. 5 x 3 = 15, and 4 x 3 = 12. ✓
Watch out. Check the scale factor with the side you know first.
Method. Multiply each length of the original by the scale factor.
- Use the scale factor. Each length is multiplied by the scale factor 2.7 x 2
- Work it out. 7 x 2 = 14.14
- Write the answer. 14
Check. 7 x 2 = 14, and 6 x 2 = 12. ✓
Watch out. Check the scale factor with the side you know first.
Method. Multiply each length of the original by the scale factor.
- Use the scale factor. Each length is multiplied by the scale factor 1.5.4 x 1.5
- Work it out. 4 x 1.5 = 6.6
- Write the answer. 6
Check. 4 x 1.5 = 6, and 10 x 1.5 = 15. ✓
Watch out. Check the scale factor with the side you know first.
Method. Find the scale factor from the widths, then use it for the height.
- Scale factor. 30 / 12 = 2.5.2.5
- New height. 8 x 2.5 = 20 cm.20 cm
- Write the answer. 20
Check. 12 x 2.5 = 30, and 8 x 2.5 = 20. ✓
Method. Multiply the length of the model by the scale factor 20.
- Scale factor. The scale 1:20 means each model length is multiplied by 20.20
- Real length. 15 x 20 = 300 cm.300 cm
- Write the answer. 300
Check. 300 / 20 = 15. ✓
Method. Test each choice against the question.
- Read what the question asks. Decide what the answer must do or show.
- Test the choice. 9 / 3 = 3, so the scale factor is 3.3 fits
- Reject the other choices. 6 would be 3 + 3, not a multiplication, and 1/3 would make the shape smaller.6 and 1/3 do not fit
- Write the answer. 3
Check. 3 is the one that fits. ✓
Method. Test each option.
- Check each option. Test each one against the rule or the calculation.
- Tick the true ones. In similar shapes the angles are equal and each length is multiplied by the same scale factor.All the angles stay the same, All lengths are multiplied by the same scale factor
- Leave the others unticked. These do not match.All the lengths stay the same, Corresponding lengths have different scale factors are not ticked
- Write the answer. All the angles stay the same, All lengths are multiplied by the same scale factor
Check. Each ticked option is true. ✓
Watch out. Lengths change in an enlargement, and different sides do not have different scale factors.
Method. Multiply the length by the scale factor.
- Multiply. 5 x 4 = 20.20
- Check. The new width is 3 x 4 = 12, so the shape is enlarged in both directions.20 cm long
- Write the answer. 20
Check. 20 / 4 = 5. ✓
Method. Use a square with side 1 cm.
- Start with the square. A 1 cm square has an area of 1 square cm.1 cm2
- Enlarge by 2. The new square has sides of 2 cm, so its area is 4 square cm.4 cm2
- Write the answer. No, because the area is multiplied by 2 x 2 = 4, not by 2.
Check. 4 = 2 x 2, not 2. ✓
Method. Find the scale factor from the matching sides, then multiply.
- Scale factor. 6 / 4 = 1.5.1.5
- Matching side. 10 x 1.5 = 15 cm.15 cm
- Write the answer. 15
Check. 4 x 1.5 = 6, so the scale factor is 1.5, and 10 x 1.5 = 15. ✓
Method. Find the scale factor for each pair of sides.
- Scale factor for the first pair. 6 / 2 = 3.3
- Scale factor for the second pair. 9 / 3 = 3.3
- Write the answer. Because 6 = 2 x 3 and 9 = 3 x 3, so both sides are multiplied by the same scale factor of 3.
Check. Both pairs have scale factor 3. ✓
Method. Multiply the plan length by the scale: each cm is 2 metres.
- Real length. 7 x 2 = 14 metres.14 metres
- Write the answer. 14
Check. 14 / 2 = 7. ✓
Chapter summary
| Skill | Method |
|---|---|
| Ratio | Compares two amounts: a:b means a parts of one to b parts of the other. |
| Simplify a ratio | Divide both parts by the same number, the highest common factor. |
| Missing amount | Find the value of one part, then multiply by the number of parts. |
| Sharing | Add the parts, divide the total by the parts, then multiply each part. |
| Unit method | Find the amount for one, then multiply. |
| Percentage | Find 1% first, then multiply to get the percentage you want. |
| Best value | Find the price of one item and compare. |
| Scale factor | New length / original length. Multiply lengths by it. |
Can you…?
- I can simplify a ratio and find a missing amount in a ratio
- I can share an amount in a given ratio and check the shares add up
- I can use the unit method to scale a recipe or a rate
- I can use a percentage to work out an original price
- I can compare prices and results fairly
- I can find a scale factor and use it to find a missing length
Key terms
- ratio
- a comparison of two or more amounts, written with a colon such as 3:5
- equivalent ratio
- a ratio that is the same as another after simplifying, such as 2:3 and 4:6
- unit method
- finding the amount for one, then multiplying to find the amount for more
- scale factor
- the number that every length in a shape is multiplied by in an enlargement
- similar shapes
- shapes with the same angles, where one is an enlargement of the other
- unit price
- the price of one item
Exercise 3Z Check-up
Non-calculatorMethod. Divide both parts of the ratio by the highest common factor.
- Find the highest common factor of both numbers. The biggest number that divides 28 and 35 is 7.HCF = 7
- Divide both parts by it. 28 / 7 = 4 and 35 / 7 = 5.4:5
- Write the answer. 4:5
Check. 4 and 5 have no common factor except 1. ✓
Watch out. Both parts must be divided by the same number, just as with a fraction.
Method. Find the value of one part, then multiply.
- Find the value of one part. 27 is 3 parts, so one part is 27 / 3.1 part = 9
- Multiply to find the other amount. There are 8 parts of circles.8 x 9 = 72
- Write the answer. 72
Check. 72 : 27 = 3:8. ✓
Watch out. Do not add the ratio numbers to the amount. Find one part first.
Method. Find the total parts, then the value of one part.
- Find the total number of parts. 2 + 7 = 9 parts.9 parts
- Find the value of one part. 180 / 9 = 20.1 part = 20
- Multiply each ratio number by one part. Leo: 2 x 20 and Ann: 7 x 2040, 140
- Write the answer. 40, 140
Check. 40 + 140 = 180. ✓
Watch out. Check that the amounts add up to the total.
Method. The total is the sum of all the parts, so the fraction is that part over the total.
- Find the total number of parts. 3 + 5 = 8.8 parts
- Write the part as a fraction of the total, in simplest form. The part is 5 out of 8 parts, which simplifies to 5/8.5/8
- Write the answer. 5/8
Check. 5 + 3 = 8. ✓
Watch out. The fraction uses the total of all parts, not the other part only.
Method. Find the amount for one person, then multiply.
- Find the amount for one. 360 ml is for 6, so one is 360 / 6.1 = 60 ml
- Multiply for the new number. Multiply by 10.60 x 10 = 600
- Write the answer. 600 ml
Check. 600 / 10 = 60, the same amount for each person. ✓
Watch out. Doubling the people does not always need double the ingredients unless you check the unit.
Method. Find 1% of the original price, then multiply by 100.
- Work out 1 per cent. 54 pounds is 90% of the price, so 1% is 54 / 90.1% = 3/5
- Work out 100 per cent. The original price is 100%, so multiply 1% by 100.3/5 x 100 = 60
- Write the answer. 60
Check. 60 less 10% is 54. ✓
Watch out. The discount is taken from the original price, so the price paid is what is left.
Method. Work out the price of one item for each pack, then compare.
- Work out the price of one item in Pack P. 320p / 4 = 80p.80p
- Work out the price of one item in Pack Q. 420p / 6 = 70p.70p
- Compare the prices of one item. The cheaper one for one item is the better value.Pack Q is cheaper for one item
- Write the answer. Pack Q
Check. Pack Q costs less for each item. ✓
Watch out. Compare the price of ONE item. A bigger pack is not always cheaper for each item.
Method. Write each result as a percentage, then compare.
- Work out the percentage for Ali. 7 out of 10 is 7 / 10 x 100.70%
- Work out the percentage for Jo. 9 out of 15 is 9 / 15 x 100.60%
- Compare the two percentages. The higher percentage is the better result.Ali is higher
- Write the answer. Ali
Check. 70% and 60% are compared fairly. ✓
Watch out. Comparing 30 with 24 is not fair when the numbers of shots are different.
Method. Divide the new length by the original length.
- Write the scale factor as a division. The scale factor is the new length divided by the original.15 / 6
- Work it out. 15 / 6 = 2.5.2.5
- Write the answer. 2.5
Check. 6 x 2.5 = 15. ✓
Watch out. A scale factor bigger than 1 makes a shape bigger.
Method. Multiply each length of the original by the scale factor.
- Use the scale factor. Each length is multiplied by the scale factor 3.4 x 3
- Work it out. 4 x 3 = 12.12
- Write the answer. 12
Check. 4 x 3 = 12, and 5 x 3 = 15. ✓
Watch out. Check the scale factor with the side you know first.
Total 19 marks. Allow about 19 minutes.
Exercise 3X Practice test questions
Non-calculatorMethod. Divide both parts of the ratio by the highest common factor.
- Find the highest common factor of both numbers. The biggest number that divides 18 and 27 is 9.HCF = 9
- Divide both parts by it. 18 / 9 = 2 and 27 / 9 = 3.2:3
- Write the answer. 2:3
Check. 2 and 3 have no common factor except 1. ✓
Watch out. Both parts must be divided by the same number, just as with a fraction.
Method. Find the value of one part, then multiply.
- Find the value of one part. 21 is 3 parts, so one part is 21 / 3.1 part = 7
- Multiply to find the other amount. There are 5 parts of pens.5 x 7 = 35
- Write the answer. 35
Check. 35 : 21 = 5:3. ✓
Watch out. Do not add the ratio numbers to the amount. Find one part first.
Method. Find the total parts, then the value of one part.
- Find the total number of parts. 1 + 3 = 4 parts.4 parts
- Find the value of one part. 84 / 4 = 21.1 part = 21
- Multiply each ratio number by one part. Ben: 1 x 21 and Eve: 3 x 2121, 63
- Write the answer. 21, 63
Check. 21 + 63 = 84. ✓
Watch out. Check that the amounts add up to the total.
Method. Find the amount for one person, then multiply.
- Find the amount for one. 150 g is for 3, so one is 150 / 3.1 = 50 g
- Multiply for the new number. Multiply by 9.50 x 9 = 450
- Write the answer. 450 g
Check. 450 / 9 = 50, the same amount for each person. ✓
Watch out. Doubling the people does not always need double the ingredients unless you check the unit.
Method. Work out the price of one item for each pack, then compare.
- Work out the price of one item in Tub A. 400p / 5 = 80p.80p
- Work out the price of one item in Tub B. 228p / 3 = 76p.76p
- Compare the prices of one item. The cheaper one for one item is the better value.Tub B is cheaper for one item
- Write the answer. Tub B
Check. Tub B costs less for each item. ✓
Watch out. Compare the price of ONE item. A bigger pack is not always cheaper for each item.
Method. Write each result as a percentage, then compare.
- Work out the percentage for Ada. 20 out of 25 is 20 / 25 x 100.80%
- Work out the percentage for Ben. 24 out of 40 is 24 / 40 x 100.60%
- Compare the two percentages. The higher percentage is the better result.Ada is higher
- Write the answer. Ada
Check. 80% and 60% are compared fairly. ✓
Watch out. Comparing 30 with 24 is not fair when the numbers of shots are different.
Method. Multiply each length of the original by the scale factor.
- Use the scale factor. Each length is multiplied by the scale factor 2.5.6 x 2.5
- Work it out. 6 x 2.5 = 15.15
- Write the answer. 15
Check. 6 x 2.5 = 15, and 4 x 2.5 = 10. ✓
Watch out. Check the scale factor with the side you know first.
Method. Divide both parts by the same number.
- Divide both parts by 4. 4 / 4 = 1 and 8 / 4 = 2.1:2
- Compare. So 4:8 = 1:2.4:8 = 1:2
- Write the answer. Because 4 and 8 can both be divided by 4, giving 1 and 2.
Check. 1:2 x 4 = 4:8. ✓
Method. Find the value of one part.
- Total parts. 5 + 4 = 9 parts.9 parts
- One part. 45 / 9 = 5.5
- Yellow. 4 parts: 4 x 5 = 20.20
- Write the answer. 20
Check. 25 red and 20 yellow make 45. ✓
Method. Test each choice against the question.
- Read what the question asks. Decide what the answer must do or show.
- Test the choice. 4 x 3 = 12, so the enlarged width is 12 cm.12 cm fits
- Reject the other choices. 7 would be 4 + 3, and 1.33 would be 4 divided by 3, which makes the shape smaller.7 cm and 1.33 cm do not fit
- Write the answer. 12 cm
Check. 12 cm is the one that fits. ✓
Method. Test each option.
- Check each option. Test each one against the rule or the calculation.
- Tick the true ones. Each ticked statement is correct: 3:6 divided by 3 gives 1:2, and 4 parts is 4 times 1 part.3:6 is the same as 1:2, A ratio of 1:4 means the second part is 4 times the first
- Leave the others unticked. These do not match.2:5 is the same as 5:2, A ratio of 1:4 means one part in 4 is the first are not ticked
- Write the answer. 3:6 is the same as 1:2, A ratio of 1:4 means the second part is 4 times the first
Check. Each ticked option is true. ✓
Watch out. 2:5 and 5:2 are different ratios, so the order matters.