# Common Denominators

In this worksheet, students use common multiples to express fractions in the same denominators. Key stage:  KS 2

Curriculum topic:   Maths and Numerical Reasoning

Curriculum subtopic:   Fractions

Difficulty level:   ### 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

 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

The lowest common multiple of 15 and 10 is 30, because 2 × 15 = 30 and 3 × 10 = 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
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
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
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
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
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
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
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
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
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
42
EDDIE SAYS
The lowest common multiple of 14 and 21 is 42.
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