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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:   Number: Fractions, Decimals and Percentages

Curriculum subtopic:   Use Common Factors

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.

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