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4.2
Calculating a percentage of
a quantity
This section will show you how to:
● calculate a percentage of a quantity
Key words
multiplier
quantity
To calculate a percentage of a quantity, you multiply the quantity by the percentage. The
percentage may be expressed as either a fraction or a decimal. When finding percentages
without a calculator, base the calculation on 10% (or 1%) as these are easy to calculate.
EXAMPLE 4
Calculate: a 10% of 54 kg b 15% of 54 kg.
a 10% is –10–1 so –10–1 of 54 kg = 54 kg ÷ 10 = 5.4 kg
b 15% is 10% + 5% = 5.4 kg + 2.7 kg = 8.1 kg
EXAMPLE 5
Calculate 12% of £80.
10% of £80 is £8 and 1% of £80 is £0.80
12% = 10% + 1% + 1% = £8 + £0.80 + £0.80 = £9.60
Using a percentage multiplier
You have already seen that percentages and decimals are equivalent so it is easier, particularly
when using a calculator, to express a percentage as a decimal and use this to do the calculation.
For example, 13% is a multiplier of 0.13, 20% a multiplier of 0.2 (or 0.20) and so on.
EXAMPLE 6
Calculate 45% of 160 cm.
45% = 0.45, so 45% of 160 = 0.45 × 160 = 72 cm
Find 52% of £460.
52% = 0.52
So, 0.52 × 460 = 239.2
This gives £239.20.
Remember to always write a money answer with 2 decimal places.
CORE 105