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Percentage change and simple interest
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Sales, pay rises and savings accounts all use percentage change. Multipliers make these quick.
Increase and decrease
Increase 60 by 15%: 15% of 60 = 9, so 60 + 9 = 69.
Or use a multiplier: 100% + 15% = 115%, so 60 × 1.15 = 69.
Decrease 250 by 30%: 100% − 30% = 70%, so 250 × 0.7 = 175.
Or use a multiplier: 100% + 15% = 115%, so 60 × 1.15 = 69.
Decrease 250 by 30%: 100% − 30% = 70%, so 250 × 0.7 = 175.
One quantity as a percentage of another
Write it as a fraction, then multiply by 100.
24 out of 80: 2480 × 100 = 30%.
Percentage change = changeoriginal × 100. From 40 to 50: 1040 × 100 = 25% increase.
Percentages can be more than 100%: 150% of 40 = 1.5 × 40 = 60.
24 out of 80: 2480 × 100 = 30%.
Percentage change = changeoriginal × 100. From 40 to 50: 1040 × 100 = 25% increase.
Percentages can be more than 100%: 150% of 40 = 1.5 × 40 = 60.
Simple interest
Simple interest is the same amount every year, worked out on the original amount.
£500 at 3% per year for 4 years: 3% of 500 = £15 per year, so 4 × 15 = £60 interest.
£500 at 3% per year for 4 years: 3% of 500 = £15 per year, so 4 × 15 = £60 interest.
A coat costs £80. Its price rises by 12%. What is the new price?
- Multiplier: 100% + 12% = 112% = 1.12
- 80 × 1.12
Answer: £89.60
For a percentage increase multiply by (1 + rate); for a decrease multiply by (1 − rate). Percentage change is change ÷ original × 100.
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