Calculating Percentages
Solve problems involving the calculation of percentages of amounts (e.g. 15% of 360) and the use of percentages for comparison
What a learner can do afterwards
- Calculate 15% of 360 by finding 10% and 5% and combining
- Compare two discounts given as percentages of different original prices
- Explain a strategy for finding any percentage of an amount using known percentage facts
The lesson
You already know how to find 10% of an amount: just divide by 10. You can use that one fact to find almost any percentage. To find 15%, split it into two easy pieces, 10% and 5%, work out each one, then add them together.
Find 15% of 360. Step 1: 10% of 360 is 360 ÷ 10 = 36. Step 2: 5% is half of 10%, so 5% of 360 is 36 ÷ 2 = 18. Step 3: add the two parts: 36 + 18 = 54. So 15% of 360 is 54.
Bag A costs £40 and has 10% off. Bag B costs £30 and has 20% off. Which bag has the bigger discount, in pounds? Bag A: 10% of £40 is £4. Bag B: 10% of £30 is £3, so 20% is £6. Bag B has the bigger discount, even though its original price is lower. The percentage alone does not tell you the answer, you need the actual amount.
When you compare percentage discounts, always work out the pound amount for each one. A bigger percentage on a smaller price can end up being a smaller saving than a smaller percentage on a bigger price.
Build any percentage from 10% and other known facts, and always calculate the actual amount before comparing discounts.
Watch it
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.