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Primary Maths · Year 5Chapter 6 · Fractions
Year 5 · Equivalent Fractions and Mixed Numbers · Lesson 6.6

Year 5: Adding and subtracting mixed numbers

Part of the Year 5 equivalent fractions and mixed numbers chapter. Chapter overview · whole chapter on one page · Download worksheet (PDF) · Answers (PDF)

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6.6

Adding and subtracting mixed numbers

Year 5
Add and subtract mixed numbers, with and without crossing a whole number, Write statements greater than 1 as a mixed number

To add mixed numbers, add the whole numbers first, then add the fractions. Put the answers together. 215 + 125: wholes 2 + 1 = 3, fractions 15 + 25 = 35. The answer is 335.

215 (blue) and 125 (orange) together fill 3 whole bars and 3 fifths.

Crossing a whole. If the fractions add to 1 or more, you must carry a whole. 234 + 134: wholes 2 + 1 = 3, fractions 34 + 34 = 64 = 124 = 112. Now add the extra whole: 3 + 112 = 412. If the denominators are different, change them to a common denominator first.

2214224234331432433444144244345S+1+ 3/4
Start at 234, add 1 to reach 334, then add 34 more: you land on 424 = 412.

Subtracting. If the fraction of the first number is big enough, subtract the wholes and the fractions separately: 456 − 116 = 3 and 46 = 346 = 323. If the fraction you are taking away is bigger, as in 315 − 135 (1 fifth is not enough to take away 3 fifths), change both numbers to improper fractions: 165 − 85 = 85 = 135. To take a mixed number from a whole number, such as 4 − 138, write 4 as 328 and 138 as 118.

Key ideas

  • Add: add the wholes, add the fractions (same denominator first), then put them together.
  • If the fractions add to 1 or more, change the fraction part to a whole and a fraction and add the whole.
  • Subtract: if the first fraction is big enough, subtract wholes and fractions separately.
  • If the fraction you take away is bigger, change both numbers to improper fractions, subtract, then change back to a mixed number.
  • Always simplify the fraction at the end.
Worked example 6.6.1
Work out 215 + 125.

Key words. To add or subtract fractions, the denominators (bottom numbers) must be the same. A common denominator is a bottom number that both denominators go into.

Method. Make sure the denominators are the same. Add the whole numbers, then add the numerators. Change the answer to a mixed number and simplify if you can.

  1. Check the denominators. To add fractions, the parts must be the same size.
    Both denominators are 5, so we can add straight away.
  2. Add the whole numbers. Add the wholes first, then deal with the fractions.
    2 + 1 = 3
  3. Add the fractions. Add the numerators. The denominator stays the same because the parts are the same size.
    15 + 25 = 1 + 25 = 35
  4. Write the answer.
    215 + 125 = 335

Check. Take away: 335 − 125 = 215. ✓

Watch out. Add the numerators only. The denominator does not get added: 1/4 + 1/4 is 2/4, not 2/8.

Worked example 6.6.2
Work out 234 + 134.

Key words. To add or subtract fractions, the denominators (bottom numbers) must be the same. A common denominator is a bottom number that both denominators go into.

Method. Make sure the denominators are the same. Add the whole numbers, then add the numerators. Change the answer to a mixed number and simplify if you can.

  1. Check the denominators. To add fractions, the parts must be the same size.
    Both denominators are 4, so we can add straight away.
  2. Add the whole numbers. Add the wholes first, then deal with the fractions.
    2 + 1 = 3
  3. Add the fractions. Add the numerators. The denominator stays the same because the parts are the same size.
    34 + 34 = 3 + 34 = 64
  4. The fraction part is 1 or more, so change it to a whole number and a fraction. 64 means 6 ÷ 4.
    64 = 124
    3 + 124 = 424
  5. Simplify the fraction part. 2 and 4 can both be divided by 2.
    24 = 2 ÷ 24 ÷ 2 = 12
    So the answer is 412.
  6. Write the answer.
    234 + 134 = 412

Check. Take away: 412 − 134 = 234. ✓

Watch out. Add the numerators only. The denominator does not get added: 1/4 + 1/4 is 2/4, not 2/8.

Worked example 6.6.3
Work out 312 + 214.

Key words. To add or subtract fractions, the denominators (bottom numbers) must be the same. A common denominator is a bottom number that both denominators go into.

Method. Make sure the denominators are the same. Add the whole numbers, then add the numerators. Change the answer to a mixed number and simplify if you can.

  1. Find a common denominator. The denominators must be the same before we compare or add. Every smaller denominator goes into 4, so 4 will do.
    4 ÷ 2 = 2, so 2 goes into 4
  2. Change the fractions to the common denominator. Multiply the top and the bottom by the same number, so each fraction keeps its value.
    312 → 12 = 1 × 22 × 2 = 24
  3. Add the whole numbers. Add the wholes first, then deal with the fractions.
    3 + 2 = 5
  4. Add the fractions. Add the numerators. The denominator stays the same because the parts are the same size.
    24 + 14 = 2 + 14 = 34
  5. Write the answer.
    312 + 214 = 534

Check. Take away: 534 − 214 = 312. ✓

Watch out. Add the numerators only. The denominator does not get added: 1/4 + 1/4 is 2/4, not 2/8.

Worked example 6.6.4
Work out 156 + 213.

Key words. To add or subtract fractions, the denominators (bottom numbers) must be the same. A common denominator is a bottom number that both denominators go into.

Method. Make sure the denominators are the same. Add the whole numbers, then add the numerators. Change the answer to a mixed number and simplify if you can.

  1. Find a common denominator. The denominators must be the same before we compare or add. Every smaller denominator goes into 6, so 6 will do.
    6 ÷ 3 = 2, so 3 goes into 6
  2. Change the fractions to the common denominator. Multiply the top and the bottom by the same number, so each fraction keeps its value.
    213 → 13 = 1 × 23 × 2 = 26
  3. Add the whole numbers. Add the wholes first, then deal with the fractions.
    1 + 2 = 3
  4. Add the fractions. Add the numerators. The denominator stays the same because the parts are the same size.
    56 + 26 = 5 + 26 = 76
  5. The fraction part is 1 or more, so change it to a whole number and a fraction. 76 means 7 ÷ 6.
    76 = 116
    3 + 116 = 416
  6. Write the answer.
    156 + 213 = 416

Check. Take away: 416 − 213 = 156. ✓

Watch out. Add the numerators only. The denominator does not get added: 1/4 + 1/4 is 2/4, not 2/8.

Worked example 6.6.5
Work out 456 − 116.

Key words. To add or subtract fractions, the denominators (bottom numbers) must be the same. A common denominator is a bottom number that both denominators go into. Key words. An improper fraction has a top number that is bigger than (or equal to) the bottom number, like 74. A mixed number is a whole number and a fraction together, like 134.

Method. Make the denominators the same, then subtract. If the fraction you are taking away is bigger, change both numbers to improper fractions first.

  1. Check the denominators. To subtract fractions, the parts must be the same size.
    Both denominators are 6, so we can subtract straight away.
  2. Subtract the whole numbers. The fraction part of the first number is big enough, so we can subtract the wholes and the fractions separately.
    4 − 1 = 3
  3. Subtract the fractions. Subtract the numerators. The denominator stays the same.
    56 − 16 = 46
  4. Simplify the fraction part. 4 and 6 can both be divided by 2.
    46 = 4 ÷ 26 ÷ 2 = 23
    So the answer is 323.
  5. Write the answer.
    456 − 116 = 323

Check. Add back: 323 + 116 = 456. ✓

Watch out. Never take the smaller number from the bigger one in each column (3⅕ − 1⅗ is not 2⅖). Change to improper fractions when you need to.

Worked example 6.6.6
Work out 578 − 234.

Key words. To add or subtract fractions, the denominators (bottom numbers) must be the same. A common denominator is a bottom number that both denominators go into. Key words. An improper fraction has a top number that is bigger than (or equal to) the bottom number, like 74. A mixed number is a whole number and a fraction together, like 134.

Method. Make the denominators the same, then subtract. If the fraction you are taking away is bigger, change both numbers to improper fractions first.

  1. Find a common denominator. The denominators must be the same before we compare or add. Every smaller denominator goes into 8, so 8 will do.
    8 ÷ 4 = 2, so 4 goes into 8
  2. Change the fractions to the common denominator. Multiply the top and the bottom by the same number, so each fraction keeps its value.
    234 → 34 = 3 × 24 × 2 = 68
  3. Subtract the whole numbers. The fraction part of the first number is big enough, so we can subtract the wholes and the fractions separately.
    5 − 2 = 3
  4. Subtract the fractions. Subtract the numerators. The denominator stays the same.
    78 − 68 = 18
  5. Write the answer.
    578 − 234 = 318

Check. Add back: 318 + 234 = 578. ✓

Watch out. Never take the smaller number from the bigger one in each column (3⅕ − 1⅗ is not 2⅖). Change to improper fractions when you need to.

Worked example 6.6.7
Work out 315 − 135.

Key words. To add or subtract fractions, the denominators (bottom numbers) must be the same. A common denominator is a bottom number that both denominators go into. Key words. An improper fraction has a top number that is bigger than (or equal to) the bottom number, like 74. A mixed number is a whole number and a fraction together, like 134.

Method. Make the denominators the same, then subtract. If the fraction you are taking away is bigger, change both numbers to improper fractions first.

  1. Check the denominators. To subtract fractions, the parts must be the same size.
    Both denominators are 5, so we can subtract straight away.
  2. The first fraction part (1) is smaller than the second (3), so we cannot take them away. Change both numbers to improper fractions. Each whole is worth 5 fifths, so the numbers become fractions over 5.
    315 = 3 × 5 + 15 = 165
    135 = 1 × 5 + 35 = 85
  3. Subtract the numerators. Now both numbers are fractions with the same denominator.
    165 − 85 = 16 − 85 = 85
  4. The answer is 1 or more, so change it to a mixed number or a whole number. 85 means 8 ÷ 5.
    85 = 135
  5. Write the answer.
    315 − 135 = 135

Check. Add back: 135 + 135 = 315. ✓

Watch out. Never take the smaller number from the bigger one in each column (3⅕ − 1⅗ is not 2⅖). Change to improper fractions when you need to.

Worked example 6.6.8
Work out 4 − 138.

Key words. To add or subtract fractions, the denominators (bottom numbers) must be the same. A common denominator is a bottom number that both denominators go into. Key words. An improper fraction has a top number that is bigger than (or equal to) the bottom number, like 74. A mixed number is a whole number and a fraction together, like 134.

Method. Make the denominators the same, then subtract. If the fraction you are taking away is bigger, change both numbers to improper fractions first.

  1. The first fraction part (0) is smaller than the second (3), so we cannot take them away. Change both numbers to improper fractions. Each whole is worth 8 eighths, so the numbers become fractions over 8.
    408 = 4 × 8 + 08 = 328
    138 = 1 × 8 + 38 = 118
  2. Subtract the numerators. Now both numbers are fractions with the same denominator.
    328 − 118 = 32 − 118 = 218
  3. The answer is 1 or more, so change it to a mixed number or a whole number. 218 means 21 ÷ 8.
    218 = 258
  4. Write the answer.
    4 − 138 = 258

Check. Add back: 258 + 138 = 4. ✓

Watch out. Never take the smaller number from the bigger one in each column (3⅕ − 1⅗ is not 2⅖). Change to improper fractions when you need to.

Worked example 6.6.9
Work out 214 − 12.

Key words. To add or subtract fractions, the denominators (bottom numbers) must be the same. A common denominator is a bottom number that both denominators go into. Key words. An improper fraction has a top number that is bigger than (or equal to) the bottom number, like 74. A mixed number is a whole number and a fraction together, like 134.

Method. Make the denominators the same, then subtract. If the fraction you are taking away is bigger, change both numbers to improper fractions first.

  1. Find a common denominator. The denominators must be the same before we compare or add. Every smaller denominator goes into 4, so 4 will do.
    4 ÷ 2 = 2, so 2 goes into 4
  2. Change the fractions to the common denominator. Multiply the top and the bottom by the same number, so each fraction keeps its value.
    12 = 1 × 22 × 2 = 24
  3. The first fraction part (1) is smaller than the second (2), so we cannot take them away. Change both numbers to improper fractions. Each whole is worth 4 quarters, so the numbers become fractions over 4.
    214 = 2 × 4 + 14 = 94
    24 = 0 × 4 + 24 = 24
  4. Subtract the numerators. Now both numbers are fractions with the same denominator.
    94 − 24 = 9 − 24 = 74
  5. The answer is 1 or more, so change it to a mixed number or a whole number. 74 means 7 ÷ 4.
    74 = 134
  6. Write the answer.
    214 − 12 = 134

Check. Add back: 134 + 12 = 214. ✓

Watch out. Never take the smaller number from the bigger one in each column (3⅕ − 1⅗ is not 2⅖). Change to improper fractions when you need to.

Watch out

Do not take the smaller fraction from the bigger one in each column. 315 − 135 is not 225 (that takes 1 fifth from 3 fifths the wrong way round). When the fraction you are taking away is bigger, change to improper fractions. Also do not leave an answer like 364. The fraction part must be less than 1.

Exercise 6F  Adding and subtracting mixed numbers

Non-calculator
1
FWork out each addition.
(a)115 + 225
(b)327 + 137
(c)218 + 438
(d)5310 + 2410
2
FWork out each addition. Each answer is a mixed number with the fraction part less than 1.
(a)234 + 134
(b)356 + 236
(c)145 + 335
(d)278 + 158
3
FWork out each addition. Change to a common denominator first.
(a)112 + 214
(b)3310 + 115
(c)223 + 156
(d)434 + 158
4
FWork out each subtraction.
(a)456 − 116
(b)678 − 238
(c)3910 − 1410
(d)545 − 215
5
FWork out each subtraction. Change to a common denominator first.
(a)578 − 234
(b)356 − 113
(c)6910 − 225
(d)434 − 112
6
FWork out each subtraction. Change to improper fractions when you need to.
(a)315 − 135
(b)414 − 234
(c)218 − 158
(d)6110 − 3710
7
FWork out each subtraction from a whole number.
(a)4 − 138
(b)5 − 214
(c)3 − 125
(d)6 − 2710
8
FUse the number line to work out 212 + 134.
2214224234331432433444144244345S+1+ 3/4
9
PSA cake recipe uses 112 cups of flour. A second recipe uses 234 cups. How much flour do they use altogether?
10
PSZainab runs 234 km. Liam runs 112 km. How much further does Zainab run?
11
PSA plank is 5 m long. Jack cuts off a piece 134 m long and then a piece 212 m long. How much of the plank is left?
12
FFind each missing number.
(a)234 + ? = 414
(b)? − 115 = 235
13
RRavi says, “315 − 135 = 225.” What mistake has Ravi made? What is the correct answer?
14
RMia says, “234 + 134 = 364.” Is her answer finished? Explain and give the correct answer.
15
RChallenge! A rectangle is 2½ cm long and 1¾ cm wide. Work out its perimeter.
2½ cm1¾ cm

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