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© Boardworks Ltd of 11 This icon indicates the slide contains activities created in Flash. These activities are not editable. For more detailed instructions, see the Getting Started presentation. This icon indicates teacher’s notes in the Notes field.

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© Boardworks Ltd of 11 This icon indicates the slide contains activities created in Flash. These activities are not editable. For more detailed instructions, see the Getting Started presentation. This icon indicates teacher’s notes in the Notes field. This is an excerpt from the “Compound Interest” presentation in Boardworks Maths for Australia, which contains 129 presentations in total.

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© Boardworks Ltd of 11 Compound percentages To find a 10% decrease, multiply by 90% or 0.9. To find a 20% decrease, multiply by 80% or 0.8. original price × 0.8 × 0.9 = original price × 0.72 A jacket is reduced by 20% in a sale. Two weeks later, the shop reduces the price by a further 10%. What is the total percentage discount? It is not 30%. 72% of 100 is equivalent to a 28% discount altogether.

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© Boardworks Ltd of 11 Jenna invests in some shares. Compound percentages After one week the value goes up by 10%. The following week they go down by 10%. Has Jenna made a loss, a gain or is she back to her original investment? Show your working.

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© Boardworks Ltd of 11 Compound percentages

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© Boardworks Ltd of 11 Jack puts $500 into a savings account with an annual compound interest rate of 5%. Compound interest As a single calculation: $500 × 1.05 × 1.05 × 1.05 × 1.05 = $ Using index notation: $500 × = $ How much will he have in the account at the end of 4 years if he doesn’t add or withdraw any money?

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© Boardworks Ltd of 11 Compound interest

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© Boardworks Ltd of 11 Compound interest Where is the best place to invest for each time period? Short term investment: 1 year Medium term investment: 3 years Long term investment: 10 years $ % annual interest Bank accountShares $ % annual interest Building Society $ % annual interest Rena is a financial advisor. She needs to work out where her client’s money would best be saved depending on how long they want to invest for.

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© Boardworks Ltd of 11 If the current population is 2345, what will it be in 5 years? Powers are used in solving problems involving repeated percentage increase and decrease. Repeated percentage change What will the population be after 10 years? The population of a village increases by 2% each year.

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© Boardworks Ltd of 11 Repeated percentage change How much will the car be worth in 2013? To decrease the value by 15%, multiply it by $ × =$6540 (to the nearest dollar) The value of a new car depreciates at a rate of 15% a year. There are 8 years between 2005 and The car costs $ in 2005.

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© Boardworks Ltd of 11 Repeated percentage change

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