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The Fractions Method - You will notice in the example that one of the 5 parts
(the unit) must be one-fifth of the total. It follows that the 2 parts is two-fifths
and the 3 parts is three-fifths. With practice it is likely that you will wish to use
this type of explanation, based on fractions, instead of the longer method of one
used in these three examples. Examples 1 to 3 are repeated below, using the
fractions method, under the topic heading Proportional Division Using
Fractions.
Example 2
A line that is 30cm long is to be divided into 3 amounts in the ratio 2:3:5. Find
the length of the longest.
1.
Add the numbers in the ratio to find the total number of parts:
2 + 3 + 5 =10
2.
Divide the total by 10 to find the size of each part:
30cm ÷ 10 = 3cm
3.
Multiply the size of one part by the number of parts in largest amount (the
5):
5 × 3cm = 15cm
Example 3
Two amounts of money are in the ratio 4:3. If the first amount is £24, what is
the second amount?
In this question, we are not given the total, and we are not asked for it either.
We just have to find the size of the other amount. We will use the method of
one.
The ratio is 4:3, and the 4 parts amount to £24. So, divide the amount for 4 parts
by 4:
4 parts
= £24
1 part
= £24 ÷ 4
=
£6
3 parts
= £6 × 3
=
£18
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