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Common Denominators

In this worksheet, students use common multiples to express fractions in the same denominators.

'Common Denominators' worksheet

Key stage:  KS 2

Curriculum topic:   Maths and Numerical Reasoning

Curriculum subtopic:   Fractions

Difficulty level:  

Worksheet Overview

QUESTION 1 of 10

Before we add or subtract fractions, we try to get them to have the same denominators.

We do this by using lowest common multiples.

 

Suppose we wish to subtract:

7 - 4
9 6

 

We look for the smallest number that 9 and 6 go into.

This will be 18, so we change both fractions into 18ths.

 

We multiply top and bottom of 7 ninths by 2:

7 = 14
9 18

 

We multiply top and bottom of 4 sixths by 3:

4 = 12
6 18

 

So we now have:

14 - 12
18 18

 

The answer is:

2 = 1
18 9

 

 

Example

When we work out this calculation, what is the lowest common denominator we should choose for both fractions?

7 - 4
15 10

 

Answer

The lowest common multiple of 15 and 10 is 30, because 2 × 15 = 30 and 3 × 10 = 30.

So the answer is 30.

When we work out this calculation, what is the lowest common denominator we should choose for both fractions?

 

3 + 5
4 6

When we work out this calculation, what is the lowest common denominator we should choose for both fractions?

 

2 + 3
3 5

When we work out this calculation, what is the lowest common denominator we should choose for both fractions?

 

1 + 3
8 4

When we work out this calculation, what is the lowest common denominator we should choose for both fractions?

 

1 + 3
5 4

When we work out this calculation, what is the lowest common denominator we should choose for both fractions?

 

3 + 3
5 10

When we work out this calculation, what is the lowest common denominator we should choose for both fractions?

 

3 + 3
4 10

When we work out this calculation, what is the lowest common denominator we should choose for both fractions?

 

3 + 5
8 6

When we work out this calculation, what is the lowest common denominator we should choose for both fractions?

 

3 + 7
10 15

When we work out this calculation, what is the lowest common denominator we should choose for both fractions?

 

3 + 7
7 8

When we work out this calculation, what is the lowest common denominator we should choose for both fractions?

 

3 + 9
14 21
  • Question 1

When we work out this calculation, what is the lowest common denominator we should choose for both fractions?

 

3 + 5
4 6
CORRECT ANSWER
12
EDDIE SAYS
The lowest common multiple of 4 and 6 is 12.
  • Question 2

When we work out this calculation, what is the lowest common denominator we should choose for both fractions?

 

2 + 3
3 5
CORRECT ANSWER
15
EDDIE SAYS
The lowest common multiple of 3 and 5 is 15.
  • Question 3

When we work out this calculation, what is the lowest common denominator we should choose for both fractions?

 

1 + 3
8 4
CORRECT ANSWER
8
EDDIE SAYS
The lowest common multiple of 8 and 4 is 8.
  • Question 4

When we work out this calculation, what is the lowest common denominator we should choose for both fractions?

 

1 + 3
5 4
CORRECT ANSWER
20
EDDIE SAYS
The lowest common multiple of 5 and 4 is 20.
  • Question 5

When we work out this calculation, what is the lowest common denominator we should choose for both fractions?

 

3 + 3
5 10
CORRECT ANSWER
10
EDDIE SAYS
The lowest common multiple of 5 and 10 is 10.
  • Question 6

When we work out this calculation, what is the lowest common denominator we should choose for both fractions?

 

3 + 3
4 10
CORRECT ANSWER
20
EDDIE SAYS
The lowest common multiple of 4 and 10 is 20.
  • Question 7

When we work out this calculation, what is the lowest common denominator we should choose for both fractions?

 

3 + 5
8 6
CORRECT ANSWER
24
EDDIE SAYS
The lowest common multiple of 8 and 6 is 24.
  • Question 8

When we work out this calculation, what is the lowest common denominator we should choose for both fractions?

 

3 + 7
10 15
CORRECT ANSWER
30
EDDIE SAYS
The lowest common multiple of 10 and 15 is 30.
  • Question 9

When we work out this calculation, what is the lowest common denominator we should choose for both fractions?

 

3 + 7
7 8
CORRECT ANSWER
56
EDDIE SAYS
The lowest common multiple of 7 and 8 is 56.
  • Question 10

When we work out this calculation, what is the lowest common denominator we should choose for both fractions?

 

3 + 9
14 21
CORRECT ANSWER
42
EDDIE SAYS
The lowest common multiple of 14 and 21 is 42.
---- OR ----

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