How_To_Be_Good_At_Math

NUMBERS • USING THE LOWEST COMMON DENOMINATOR 51

Using the lowest

common denominator

When we need to rewrite fractions to give them the same

denominator, the simplest way is to use something called

the lowest common denominator.

Let’s compare the fractions^3 ⁄ 4 and^7 ⁄ 10. To do this, we’ll

change them so they have the same denominator.

Let’s look for the lowest common multiple of

the two denominators – we learned about

common multiples on page 31. We can use

number lines to find that 20 is the lowest

common multiple of 4 and 10. Now let’s rewrite

the fractions with 20 as their denominator.

To do this, we work out how

many times each fraction’s

original denominator goes into

20, and multiply both the numerator

and denominator by that number.

so

0

0

4 8 12

10

MULTIPLES OF 4

MULTIPLES OF 10

16

Who’s best in the test?

In a maths test,^4 ⁄ 5 of Zeek’s answers were correct.

Wook got^5 ⁄ 6 of them correct. Can you work out

who got most answers right? Here’s a handy hint

• start by finding the lowest common denominator!

Answer on page 319

The denominator of

(^3) ⁄ 4 goes into 20 five
times so we multiply
both numbers by 5
The denominator goes into 20 two times
so we multiply both numbers by 2
15
20
14

20
3
4
7
10
3
4
7
? 10
Now that both denominators are the same, it’s
easy to compare the numerators. We see that
(^15) ⁄ 20 is greater than (^14) ⁄ 20 , so (^3) ⁄ 4 is greater than (^7) ⁄ 10.
20
20
3
4
15
20
x 5
x 5
7
10
14
20
x 2
x 2
= =
TRY IT OUT
Zeek
Wook
050_051_Comparing_Non_Unit_Fractions.indd 51 29/02/2016 14:09