Starter quiz
- Which statement is the equivalent of "Decrease 900 by 21%"?
- Find 21% of 900
- Find 89% of 900
- Find 79% of 900 ✓
- Subtract 21 from 900
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- If I increase £45 to £54, what is the percentage gain?
- '20' ✓
- What is the amount of simple 1.8% interest earned on an investment of £12 000 over 3 years?
- '648' ✓
- What is the amount of compound 1.6% interest earned on an investment of £1900 over 9 years?
- '291.78' ✓
- The cost of a music subscription increases by 1.9% each month for 8 months. It now costs £11.16. What was the original price?
- '£9.60' ✓
- What is the amount of simple 2.5% interest earned on an investment of £15 000 over 5 years?
- '1875' ✓
Exit quiz
- The value of a car depreciates by 4% each year. If the car is worth £10 000 when purchased, how much is it worth after 3 years?
- '8847.36' ✓
- The value of a poor investment depreciates by 25% each year. If the investment was worth £30 000 when purchased how much is it worth after 3 years?
- '12656.25' ✓
- The value of a poor investment depreciates by 8% each year. If the investment was worth £20 000 when purchased how much is it worth after 3 years?
- '15573.76' ✓
- A good investment gains value by 12% each year. If the investment was worth £20 000 when purchased, how much is it worth after 3 years?
- '28098.56' ✓
- The value of a car is £8671.80 after 4 years of ownership. It was bought for £10 000. What is the percentage decrease per year?
- '3.5%' ✓
- The value of a poor investment is £18 544.33 after 5 years of ownership. It was bought for £20 000. What is the percentage decrease in value per year?
- '1.5%' ✓
Worksheet
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Presentation
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Video
Lesson Details
Key learning points
- If the compound interest rate is fixed but unknown, you can calculate it.
- To calculate it you need to know the start and end values, as well as the period of time.
- The interest rate is interpreted based on the period of time.
- The period of time could be per day/week/month/year etc.
Common misconception
When using multipliers, pupils can mistake a decimal for the percentage decrease. e.g. 0.52 is a 52% decrease, rather than recognising a 48% decrease.
Reminding students that a decimal multiplier greater than 1 means an increase, and a decimal multiplier less than 1 means a decrease. The latter requires a subtraction from 1 or 100%.
Keywords
Rate of interest - The rate of interest is the percentage by which an amount increases.
Compound interest - Compound interest is the interest calculated on the original amount and the interest accumulated over the previous period.
Exponential form - When a number is multiplied by itself multiple times, it can be written more simply in exponential form.
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