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Primary Maths · Year 2Chapter 2 · Adding and subtracting
Year 2 · Chapter 2

Adding and subtracting

By the end of this chapter you will be able to

  • Know the facts to 20, and use them to find related facts up to 100
  • Add and take away ones, and tens, from a two-digit number
  • Add and subtract two two-digit numbers, using a written method
  • Add three one-digit numbers, looking for two that make 10 first
  • Use the inverse to check an answer, and find a missing number
  • Solve word problems by adding or taking away

You should already know

  • Adding and taking away within 20 (Year 1)
  • Numbers to 100, tens and ones (Chapter 1)
  • Counting in tens (Chapter 1)

Where this leads

  • Multiplying and dividing (Chapter 3)
  • Measures and money, where we add and take away (Chapters 5 and 6)

National Curriculum for England, Key Stage 1 mathematics, Year 2. Content: Recall and use addition and subtraction facts to 20 fluently, Derive and use related addition and subtraction facts up to 100, Add and subtract a two-digit number and ones, Add and subtract a two-digit number and tens, Add and subtract two two-digit numbers, Add three one-digit numbers, Show that addition can be done in any order and subtraction cannot, Use the inverse to check calculations and solve missing number problems, Solve problems with addition and subtraction using objects, pictures and written methods.

Exercise P  Do you remember?

Non-calculator
1
FWork out 8 + 6.
2
FCount on 3 from 14. What number do you reach?
1011121314151617181920
3
FWhat is 10 more than 45?
123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100
4
FWork out 9 − 4.
4?9
5
FHow many tens and ones are in 58?
2.1

Facts to 20 and related facts

Year 2
Recall and use addition and subtraction facts to 20 fluently, Derive and use related addition and subtraction facts up to 100
Video lesson: Facts to 20 and related factsComing soon
Counting the flowers in the garden.

Some sums are so well known that we say the answer at once. These are facts to 20.

If you know one fact, you know more. Here is an example.

3 + 4 = 7. So 30 + 40 = 70, because 3 tens and 4 tens make 7 tens.

3 tens and 4 tens. Only the tens change.

Key ideas

  • Know the facts to 20 so well that you do not count.
  • Use a fact you know to find a sum or a take-away.
  • If 3 + 4 = 7, then 30 + 40 = 70.
  • Check a take-away by adding back: 70 − 30 = 40, so 40 + 30 = 70.
Worked example 2.1.1
Work out 7 + 6.

Key words. A fact is a sum that you know. Know the facts to 20 without counting.

Method. Use the way that makes the fact easy to hold in your head.

  1. Start with the bigger number. 7 is the bigger number, so start there.
    7
  2. Make 10. 7 needs 3 more to make 10.
    7 + 3 = 10
  3. Add what is left. 3 of the 6 makes 10, so 3 is left.
    10 + 3 = 13

Check. Take 6 away from 13. You get 7. ✓

Watch out. Do not count from one again. Start at the bigger number.

Worked example 2.1.2
Work out 14 − 6.
6?14

Key words. Related facts use the same three numbers.

Method. Think of the adding fact that goes with it.

  1. Look at the take-away. Take 6 away from 14.
    14 − 6
  2. Think of the addition that matches. 6 + ? = 14 is the same fact.
    6 + 8 = 14
  3. Write the answer. The missing part is 8.
    8

Check. 8 + 6 = 14. ✓

Watch out. Take away and add are linked. Use the adding fact to check.

Worked example 2.1.3
If 6 + 4 = 10, what is 60 + 40?

Key words. A ten is a bundle of ten ones.

Method. The same fact works with tens. Each number is 10 times bigger, and so is the answer.

  1. Look at the fact 6 + 4 = 10. This is the fact we know.
    6 + 4 = 10
  2. Make each number 10 times bigger. 6 becomes 60 and 4 becomes 40.
    6 tens = 60, 4 tens = 40
  3. Work out the answer. The answer is 10 tens, so it is 100.
    60 + 40 = 100

Check. 10 tens is 100. ✓

Watch out. Use the fact you know. Put a zero on each number and on the answer.

Watch out

Watch out: 3 + 4 = 7, so 30 + 40 = 70. The answer is not 37.

Exam tip

Grown-up tip. Ask: “If 3 + 4 = 7, what is 30 + 40?” Let your child answer, then check with 3 bundles of ten and 4 bundles of ten.

Exercise 2A  Facts to 20 and related facts

Non-calculator
1
FWork out 9 + 5.
2
FWork out 8 + 8.
3
FWork out 16 − 7.
7?16
4
FFind the missing number: 8 + ? = 15
8?15
5
FFind the missing number: ? + 6 = 13
?613
6
FIf 4 + 5 = 9, what is 40 + 50?
7
FIf 9 − 3 = 6, what is 90 − 30?
8
FIf 8 − 5 = 3, what is 80 − 50?
9
FUse a fact you know to find each answer.
5?12
(a)7 + 4
(b)12 − 5
(c)6 + 9
10
PSLeo has 7 books on one shelf and 9 books on another. How many books does he have altogether?
11
RBen says 3 tens and 6 tens make 9 tens. Is he right? Explain.
2.2

Adding and taking away ones

Year 2
Add and subtract a two-digit number and ones, using objects, pictures and written methods
Video lesson: Adding and taking away onesComing soon
Counting biscuits in the kitchen.

A two-digit number has tens and ones. To add or take away ones, only the ones change.

Look at 34 + 5. The tens stay at 3 tens. The ones make 4 + 5 = 9. So 34 + 5 = 39.

Sometimes the ones make 10 or more. Then we make a new ten.

The number 34 has 3 tens and 4 ones.

Key ideas

  • Add the ones to the ones.
  • The tens stay the same unless we make a new ten.
  • If the ones make 10 or more, make a new ten.
  • Take away the ones, then keep the tens.
Worked example 2.2.1
Work out 34 + 5.

Key words. An exchange swaps 1 ten for 10 ones, or 10 ones for 1 ten.

Method. Work with the ones first. Then keep the tens.

  1. Look at the ones. 4 ones and 5 ones make 9 ones.
    4 + 5 = 9
  2. Keep the tens the same. Adding ones does not change the tens.
    3 tens stay the same
  3. Put the parts together. 30 and 9 make 39.
    30 + 9 = 39

Check. Take 5 away from 39. You get 34. ✓

Watch out. Work with the ones first. If the ones do not fit, take a ten and make ten ones.

Worked example 2.2.2
Work out 38 + 5. The ones make a new ten.

Key words. An exchange swaps 1 ten for 10 ones, or 10 ones for 1 ten.

Method. Work with the ones first. Then keep the tens.

  1. Look at the ones. 8 + 5 = 13. That is 10 or more ones, so make a new ten.
    8 + 5 = 13
  2. Make a new ten. 13 ones is 1 ten and 3 ones.
    13 = 1 ten and 3 ones
  3. Add the new ten to the tens. 3 tens and 1 more ten make 4 tens.
    40
  4. Put the parts together. 40 and 3 make 43.
    40 + 3 = 43

Check. Take 5 away from 43. You get 38. ✓

Watch out. Work with the ones first. If the ones do not fit, take a ten and make ten ones.

Worked example 2.2.3
Work out 42 − 5. You cannot take 5 ones from 2 ones.

Key words. An exchange swaps 1 ten for 10 ones, or 10 ones for 1 ten.

Method. Work with the ones first. Then keep the tens.

  1. Look at the ones. You cannot take 5 from 2, so take 1 ten.
    2 is less than 5
  2. Exchange 1 ten for 10 ones. 4 tens becomes 3 tens and 12 ones.
    2 + 10 = 12
  3. Take the ones away. 12 − 5 = 7
    12 − 5 = 7
  4. Put the parts together. 30 and 7 make 37.
    30 + 7 = 37

Check. Add 5 to 37. You get 42. ✓

Watch out. Work with the ones first. If the ones do not fit, take a ten and make ten ones.

Watch out

Watch out: 38 + 5 is 43, not 33. The ones make 13, so we make a new ten.

Exam tip

Grown-up tip. Use 3 bundles of ten pasta and 4 loose pieces. Ask: “Add 5 more single pieces. How many now?” Then ask: “Did we make a new bundle?”

Exercise 2B  Adding and taking away ones

Non-calculator
1
FWork out 23 + 4.
2
FWork out 56 − 3.
3
FWork out 37 + 2.
4
FWork out 48 + 6.
5
FWork out 65 − 7.
6
FWork out each sum.
(a)24 + 3
(b)39 + 5
(c)62 + 8
7
FWork out each take-away sum.
(a)46 − 4
(b)53 − 8
(c)90 − 6
8
PSNoah has 36 stickers. He gets 4 more. How many stickers does he have now?
9
PSA bus has 46 people on it. 7 people get off. How many people are on the bus now?
10
RRavi says 47 + 5 = 52. Is he right? Explain how you know.
11
ROliver says 25 − 3 = 23. Is he right? Explain how you know.
2.3

Adding and taking away tens

Year 2
Add and subtract a two-digit number and tens
Video lesson: Adding and taking away tensComing soon
Packs of ten in the shop.

Adding or taking away tens changes only the tens digit.

Look at 34 + 30. The ones stay 4. The tens are 3 + 3 = 6 tens. So 34 + 30 = 64.

On a 100 square, adding 10 goes down one row. Taking away 10 goes up one row.

123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100
Start at 34. Adding 30 moves down three rows.

Key ideas

  • Adding or taking away tens changes only the tens digit.
  • Adding 10 moves down one row on the 100 square.
  • Taking away 10 moves up one row.
  • The ones digit stays the same.
Worked example 2.3.1
Work out 34 + 30.
123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100

Key words. Adding tens means adding groups of ten.

Method. Only the tens change. The ones stay the same.

  1. Look at the tens. 34 has 3 tens. We add 3 tens.
    3 tens + 3 tens
  2. Work out the new tens. 3 + 3 = 6, so there are 6 tens.
    6 tens = 60
  3. Keep the ones the same. The ones digit is 4 and it does not change.
    4 ones stay
  4. Put the parts together. 60 and 4 make 64.
    60 + 4 = 64

Check. 34 + 30 = 64. Count in tens to check. ✓

Watch out. Do not change the ones digit. 34 becomes 64, not 35.

Worked example 2.3.2
Work out 58 − 20.
123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100

Key words. Adding tens means adding groups of ten.

Method. Only the tens change. The ones stay the same.

  1. Look at the tens. 58 has 5 tens. We take away 2 tens.
    5 tens − 2 tens
  2. Work out the new tens. 5 − 2 = 3, so there are 3 tens.
    3 tens = 30
  3. Keep the ones the same. The ones digit is 8 and it does not change.
    8 ones stay
  4. Put the parts together. 30 and 8 make 38.
    30 + 8 = 38

Check. 58 − 20 = 38. Count in tens to check. ✓

Watch out. Do not change the ones digit. 58 becomes 38, not 57.

Worked example 2.3.3
Work out 45 + 40.

Key words. Adding tens means adding groups of ten.

Method. Only the tens change. The ones stay the same.

  1. Look at the tens. 45 has 4 tens. We add 4 tens.
    4 tens + 4 tens
  2. Work out the new tens. 4 + 4 = 8, so there are 8 tens.
    8 tens = 80
  3. Keep the ones the same. The ones digit is 5 and it does not change.
    5 ones stay
  4. Put the parts together. 80 and 5 make 85.
    80 + 5 = 85

Check. 45 + 40 = 85. Count in tens to check. ✓

Watch out. Do not change the ones digit. 45 becomes 85, not 46.

Watch out

Watch out: 34 + 30 is 64, not 37. Only the tens change.

Exam tip

Grown-up tip. Use bundles of ten pasta or straws. Ask: “Add 3 bundles to 4 loose pieces. How many bundles now? How many loose pieces?”

Exercise 2C  Adding and taking away tens

Non-calculator
1
FWork out 36 + 20.
123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100
2
FWork out 58 − 30.
123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100
3
FWork out 45 + 40.
123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100
4
FWork out 26 + 50.
123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100
5
FWork out 90 − 60.
123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100
6
FWork out each sum.
(a)23 + 10
(b)47 − 20
(c)62 + 30
7
FWork out each sum.
(a)19 + 60
(b)84 − 40
(c)31 + 40
8
PSMei has 42 stickers. Her friend gives her 30 more. How many stickers does Mei have now?
MeiFriend
9
PSThere are 83 cars in the car park. 20 cars leave. How many cars are left?
10
PSSam has 64 counters. He gives 40 away. How many counters does he have left?
11
RTom says 36 + 20 = 46. Is he right? Explain how you know.
123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100
12
RWhich is more: 45 + 30 or 60 + 10? Explain how you know.
123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100
2.4

Adding and subtracting two two-digit numbers

Year 2
Add and subtract two two-digit numbers, using a written method
Video lesson: Adding and subtracting two two-digit numbersComing soon
Counting the animals on the farm.

To add two two-digit numbers, split each number into tens and ones.

Add the tens. Then add the ones. Then put the two answers together.

For 36 + 27: 30 + 20 = 50. 6 + 7 = 13. 50 + 13 = 63.

A column layout keeps the tens above the tens. The ones go above the ones.

36 and 27, each split into tens and ones.

Key ideas

  • Tens go with tens. Ones go with ones.
  • Split each number into tens and ones.
  • If the ones make 10 or more, make a new ten.
  • Check a sum by taking one number away from the total.
Worked example 2.4.1
Work out 36 + 27 in columns.
36
+27

Key words. A written method splits the numbers into tens and ones.

Method. Split each number into tens and ones. Work with the tens, then the ones.

  1. Add the ones. 6 + 7 = 13. That is 10 or more, so make a new ten.
    6 + 7 = 13
  2. Write the ones and carry the ten. 13 ones is 1 ten and 3 ones. Write 3; carry 1 ten.
    13 = 1 ten and 3 ones
  3. Add the tens, with the new ten. 3 tens + 2 tens + 1 ten = 6 tens.
    30 + 20 + 10 = 60
  4. Put the parts together. 60 and 3 make 63.
    60 + 3 = 63

Check. Take 27 away from 63. You get 36. ✓

Watch out. Tens go with tens, and ones go with ones. Do not mix them up.

Worked example 2.4.2
Work out 58 − 23 in columns.
58
−23

Key words. A written method splits the numbers into tens and ones.

Method. Split each number into tens and ones. Work with the tens, then the ones.

  1. Take away the tens. 5 tens − 2 tens = 3 tens.
    50 − 20 = 30
  2. Take away the ones. 8 ones − 3 ones = 5 ones.
    8 − 3 = 5
  3. Put the parts together. 30 and 5 make 35.
    30 + 5 = 35

Check. Add 23 to 35. You get 58. ✓

Watch out. Tens go with tens, and ones go with ones. Do not mix them up.

Worked example 2.4.3
Work out 45 + 38. The ones make a new ten.
1
45
+38

Key words. A written method splits the numbers into tens and ones.

Method. Split each number into tens and ones. Work with the tens, then the ones.

  1. Add the ones. 5 + 8 = 13. That is 10 or more, so make a new ten.
    5 + 8 = 13
  2. Write the ones and carry the ten. 13 ones is 1 ten and 3 ones. Write 3; carry 1 ten.
    13 = 1 ten and 3 ones
  3. Add the tens, with the new ten. 4 tens + 3 tens + 1 ten = 8 tens.
    40 + 30 + 10 = 80
  4. Put the parts together. 80 and 3 make 83.
    80 + 3 = 83

Check. Take 38 away from 83. You get 45. ✓

Watch out. Tens go with tens, and ones go with ones. Do not mix them up.

Watch out

Watch out: when the ones make 10 or more, carry a new ten to the tens. 6 + 7 = 13 makes 1 new ten.

Exam tip

Grown-up tip. Write the sum on squared paper, one digit in each box. Ask: “How many ones? Do we need a new ten?”

Exercise 2D  Adding and subtracting two two-digit numbers

Non-calculator
1
FWork out 34 + 25 in columns.
34
+25
??
2
FWork out 52 + 31 in columns.
52
+31
??
3
FWork out 46 + 17 in columns.
46
+17
??
4
FWork out 68 − 35 in columns.
68
−35
??
5
FWork out 85 − 27 in columns.
85
−27
??
6
FWork out 57 + 36 in columns.
57
+36
??
7
FWork out 64 + 28 in columns.
64
+28
??
8
FWork out each sum or take-away.
(a)23 + 46
(b)75 − 32
(c)38 + 44
9
PSA shop sold 36 red apples and 27 green apples. How many apples were sold altogether?
red 36green 27?
10
PSLeo has 84 marbles. He gives 39 marbles to Ravi. How many marbles does Leo have left?
39?84
11
RSam says 47 + 28 = 65. Is he right? Explain how you know.
47
+28
2.5

Adding three numbers and the order of numbers

Year 2
Add three one-digit numbers, Show that addition can be done in any order and subtraction cannot
Video lesson: Adding three numbers and the order of numbersComing soon
Three groups of things in the park.

We can add three one-digit numbers. Look for two numbers that make 10 first.

For 4 + 6 + 3: 4 + 6 = 10. Then 10 + 3 = 13.

Adding works in any order: 5 + 2 is the same as 2 + 5. Taking away does not.

Three groups of stars: 4, 6 and 3.

Key ideas

  • Look for two numbers that make 10 first.
  • Then add the last number.
  • Adding can be done in any order.
  • Taking away cannot be done in any order.
Worked example 2.5.1
Work out 4 + 6 + 3.

Method. Find two numbers that make 10 first. Then add the last number.

  1. Look for two numbers that make 10. 4 and 6 make 10.
    4 + 6 = 10
  2. Add the last number. Then add the number that is left.
    10 + 3 = 13
  3. Write the answer.
    13

Check. Add them in a different order to check: 3 + 6 + 4 = 13. ✓

Watch out. Keep the numbers in your head. Do not forget the last number.

Worked example 2.5.2
Work out 3 + 6 + 8. No two of them make 10, so add two first.

Method. Add two numbers first. Then add the last number.

  1. Add the first two numbers. Start with the first two numbers.
    3 + 6 = 9
  2. Add the last number. Add 8 to the answer so far.
    9 + 8 = 17
  3. Write the answer.
    17

Check. Add them in a different order to check: 8 + 6 + 3 = 17. ✓

Watch out. Keep the numbers in your head. Do not forget the last number.

Worked example 2.5.3
Is 5 + 2 the same as 2 + 5?

Method. Swap the two numbers. Check that the answer is the same.

  1. Add the numbers in one order. 5 and 2 make 7.
    5 + 2 = 7
  2. Swap the numbers and add again. Now the bigger number comes first.
    2 + 5 = 7
  3. Compare the answers. Both answers are the same, so the order does not matter.
    Yes. 5 + 2 = 2 + 5

Check. Both orders make 7. ✓

Watch out. Adding is in any order. Taking away is NOT in any order.

Watch out

Watch out: 9 − 4 is 5. But 4 − 9 cannot be done. Taking away is not in any order.

Exam tip

Grown-up tip. Use three cups of pasta. Ask: “Which two cups make 10?” Then ask: “Does it matter which one we add first?”

Exercise 2E  Adding three numbers and the order of numbers

Non-calculator
1
FWork out 2 + 5 + 3.
2
FWork out 6 + 3 + 4.
3
FWork out 7 + 1 + 9.
4
FWork out 5 + 8 + 5.
5
FWork out 3 + 6 + 8.
6
FWork out each sum.
(a)4 + 3 + 6
(b)9 + 2 + 8
(c)5 + 5 + 4
7
PSAmira has 4 red pens, 5 blue pens and 6 green pens. How many pens does she have?
8
PSThree bags have 3, 7 and 8 sweets. How many sweets are there altogether?
9
RIs 8 + 6 the same as 6 + 8? Explain how you know.
10
RPriya says 9 − 4 is the same as 4 − 9. Is she right? Explain.
11
RWhich one can we NOT work out with objects: 7 − 3 or 3 − 7?
2.6

Inverse checks and missing numbers

Year 2
Use the inverse to check calculations and solve missing number problems, Solve problems with addition and subtraction using objects, pictures and written methods
Video lesson: Inverse checks and missing numbersComing soon
Waiting at the bus stop.

Adding and taking away undo each other. We call this the inverse.

If 46 + 27 = 73, then 73 − 27 must give 46. Use this to check your answer.

A missing number is shown with a box. Do the opposite to find it.

274673
46 and 27 make 73.

Key ideas

  • Check an addition by taking away one part from the total.
  • Check a take-away by adding back.
  • To find a missing number, do the opposite.
  • Put the number in the box to check the sum.
Worked example 2.6.1
Check 46 + 27 = 73. Use 73 − 27.
274673

Method. Check an addition by taking away one of the parts from the total.

  1. Write the check. Take 27 away from 73, the total.
    73 − 27
  2. Work it out. The answer should be the first number, 46.
    73 − 27 = 46
  3. Compare. It matches the first number, so the addition is correct.
    46 = 46

Check. 46 + 27 = 73, and 73 − 27 = 46. ✓

Watch out. The inverse of adding is taking away. Use it to check.

Worked example 2.6.2
Find the missing number: ? + 15 = 40
15?40

Method. The box is a number we do not know. Do the opposite to find it.

  1. Read the sum with the box. The box stands for the missing number.
    ? + 15 = 40
  2. Do the opposite to find it. To undo adding 15, take 15 away from 40.
    40 − 15 = 25
  3. Check it in the sum. Put the number in the box and see if it works.
    25 + 15 = 40

Check. 25 + 15 = 40. ✓

Watch out. Adding undoes taking away. Taking away undoes adding.

Worked example 2.6.3
Find the missing number: 50 − ? = 28
28?50

Method. The box is a number we do not know. Do the opposite to find it.

  1. Read the sum with the box. The box stands for the missing number.
    50 − ? = 28
  2. Do the opposite to find it. The box is the part to take away. Take 28 from 50.
    50 − 28 = 22
  3. Check it in the sum. Put the number in the box and see if it works.
    50 − 22 = 28

Check. 50 − 22 = 28. ✓

Watch out. Adding undoes taking away. Taking away undoes adding.

Watch out

Watch out: for 50 − ? = 28, the box is 22. Do not add 50 and 28.

Exam tip

Grown-up tip. Write 46 + 27 = 73 on paper. Ask: “What is 73 take away 27?” Then check it together.

Exercise 2F  Inverse checks and missing numbers

Non-calculator
1
FCheck 46 + 27 = 73. Take 27 away from 73. What do you get?
274673
2
FFind the missing number: ? + 15 = 40
15?40
3
FFind the missing number: 50 − ? = 28
28?50
4
FFind the missing number: ? − 18 = 36
3618?
5
FFind the missing number: 38 + ? = 70
38??
6
FFind the missing number: 64 − ? = 40
?4064
7
FCheck each sum. Is it right? Answer Yes or No.
(a)75 − 39 = 36. Check by adding.
(b)40 + 27 = 58. Check by taking away.
(c)58 − 23 = 35. Check by adding.
8
PSNoah has some stickers. He gets 25 more. Now he has 60. How many did he have at the start?
25 more?60
9
PSJack had 80 cards. Some were given away. Now he has 45. How many cards were given away?
?4580
10
RLeo says the missing number in ? + 24 = 50 is 74. Is he right? Explain.
24?50
11
RWhich number goes in both boxes? ? + 10 = 35 and 35 − ? = 10.
?1035

Chapter summary

SkillMethod
Facts to 20 and related factsKnow the facts to 20. Use 3 + 4 = 7 to find 30 + 40 = 70.
OnesAdd or take away the ones. Keep the tens. Make a new ten if the ones go over 10.
TensOnly the tens change. The ones stay the same.
Two two-digit numbersSplit into tens and ones. Add the tens, add the ones, then put them together.
Three one-digit numbersLook for two that make 10 first. Then add the last number.
OrderAddition can be done in any order. Subtraction cannot.
Inverse and missing numbersCheck an addition by taking away. Find a missing number by doing the opposite.

Can you…?

  • I can use facts to 20 to find related facts up to 100
  • I can add and take away ones and tens from a two-digit number
  • I can add and subtract two two-digit numbers in columns
  • I can add three one-digit numbers by making 10 first
  • I know addition can be done in any order, and subtraction cannot
  • I can check an answer with the inverse, and find a missing number

Key terms

fact
a sum or take-away we know without counting
related facts
facts that use the same numbers, such as 3 + 4 = 7 and 30 + 40 = 70
inverse
the opposite operation: take away undoes adding
exchange
swap 1 ten for 10 ones, or 10 ones for 1 ten
column method
a written sum with tens above tens and ones above ones
missing number
a number we do not know yet, shown by a box
altogether
the total, found by adding

Exercise 2Z  Check-up

Non-calculator
1
Work out 8 + 7.
2
Work out 14 − 9.
9?14
3
If 3 + 6 = 9, what is 30 + 60?
4
Work out 47 + 6.
5
Work out 63 − 8.
6
Work out 54 + 30.
123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100
7
Work out 47 + 35 in columns.
47
+35
??
8
Work out 72 − 48 in columns.
72
−48
??
9
Work out 5 + 7 + 5.
10
Find the missing number: ? + 16 = 39
16?39

Total 14 marks. Allow about 14 minutes.

Exercise 2X  End-of-chapter quiz

Non-calculator
1
FWork out 6 + 9.[1 mark]
2
FWork out 17 − 8.[1 mark]
8?17
3
FIf 7 + 2 = 9, what is 70 + 20?[1 mark]
4
FWork out 59 + 6.[1 mark]
5
FWork out 86 − 40.[1 mark]
123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100
6
FWork out 38 + 46 in columns.[2 marks]
38
+46
??
7
FWork out 81 − 46 in columns.[2 marks]
81
−46
??
8
PSWork out 2 + 8 + 6.[1 mark]
9
PSFind the missing number: ? − 15 = 40[2 marks]
4015?
10
RSam says 53 + 29 = 72. Is he right? Explain how you know.[2 marks]
53
+29
← Chapter 1: Place value to 100

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