Percentage increase, decrease and simple interest

Mathematics · Ratio & Proportion · ages 12-14

Name ______________________ Date ____________

To decrease an amount by 20%, what decimal do you multiply by?
- 0.20
- 0.80
- 1.20
To increase an amount by 15%, what decimal do you multiply by?
- 0.15
- 1.15
- 1.5
Simple interest is calculated only on the original amount, not on any interest already earned.
Circle one: True False
A 20% discount leaves a sale price of £64. What was the original price?
Answer: ______________
After a 20% discount the price is £64. What was it before?
- £80
- £51.20
- £84
- £76.80
Calculate a 15% increase on £240 using a decimal multiplier.
Answer: ______________
£180 is decreased by 10%. What is the new amount?
Answer: ______________
How much simple interest does £500 earn at 3% per year for 4 years?
Answer: ______________
After a 25% discount the price is £90. What was the original price?
- £120
- £112.50
- £115
- £67.50
Priya wants to find the original price of a jumper that sold for £64 after a 20% discount. She calculates £64 × 0.80. What mistake did she make?
- She should have divided by 0.80, not multiplied
- She should have used 0.20 instead of 0.80
- She should have added 20% to 64

Answer key

For grown-ups. Fold this page away before handing over the rest.

0.80 · 80% is 100% (the original) minus 20% (the decrease), which as a decimal is 0.80.
1.15 · 115% is 100% (the original) plus 15% (the increase), which as a decimal is 1.15.
True · Simple interest stays the same every year because it's always worked out on the original amount.
80 · 64 / 0.8 = 80.
£80 · £64 is 80%, so the whole is 64 / 0.8 = £80.
276 · 240 x 1.15 = 276.
162 · 180 x 0.9 = 162.
60 · 500 x 0.03 x 4 = 60.
£120 · £90 is 75%, so the whole is 90 / 0.75 = £120.
She should have divided by 0.80, not multiplied · Multiplying by 0.80 finds a smaller amount, not the original. Dividing by 0.80 undoes the discount instead.