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Before we compare fractions, we try to get them to have the same denominators.

We do this by using lowest common multiples.

 

Suppose we wish to compare:

7  and  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  and  12
18 18

 

Clearly

14  >  12
18 18

 

So

7  >  4
9 6

 

 

Example

Which is bigger?

12  or  11
8 6

 

Answer

The lowest common multiple of 8 and 6 is 24, because 3 × 8 = 24 and 4 × 6 = 24.

We multiply top and bottom of 12 eighths by 3:

12  =  36
8 24

 

We multiply top and bottom of 11 sixths by 4:

11  =  44
6 24

 

Clearly

44

 > 

36
24 24

 

So

11 is bigger
6

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