Percentages are an important skill to master. You'll come across them in real life all the time - wage increases, inflation rates, interest rates etc. No wonder they form a key part of KS3 Maths. In this quiz we look at calculating increases and decreases in term of percentages.

You will often have to find a value when an amount increases or decreases by a percentage. There is more than one way of working it out but perhaps the easiest is to multiply by a decimal number. Say that something you want to buy for £200 has a 5% decrease in price. To work out the new price multiply £200 x 0.95 (95% of the previous price). If the price has increased by 5% then multiply £200 by 1.05 (105% of the previous price). There are other methods you can use so don't feel that you must follow my example!

Try this quiz about increases and decreases in percentages to get some practise - whatever the method you use!

1.

In a sale, all the prices are reduced by 20%. Find the sale price of a washing machine which originally cost £299.

£60

£100

£176.30

£239.20

One way is 20 / 100 x £299 = £59.80. Then subtract £59.80 from £299 to get £239.20

2.

Thomas earns £22,500 per year. He receives a pay rise of 4.4%. How much does he now earn?

£23,000

£23,400

£23,450

£23,490

4.4% of 22,500 is 990. To work this out you could multiply 22,500 x 0.044 (4.4 hundredths)

3.

Harold owns a 14% stake in a company worth £250,000. How much is Harold's stake worth?

£35,000

£40,000

£45,000

£50,000

To find 14% multiply by 0.14

4.

Simon bought his computer for £230. A year later he sold it for 20% less than he paid for it. How much was it sold for?

£164

£184

£204

£224

To work out 20% just divide by 5

5.

To find the result of decreasing a number by 7% you multiply it by .......

0.9

0.93

1.07

1.7

Percentages are easily converted to decimals. Just divide by 100 (or move digits)

6.

A train's top speed is 140 mph. After a service, its top speed increases by 12%. What is the new top speed?

150 mph

156 mph

156.4 mph

156.8 mph

A 12% increase is a multiplier of 1.12. 1.12 x 140 = 156.8

7.

To find the result of increasing a number by 66% you multiply it by .......

1.16

1.6

1.66

34

Calculations are much easier if a multiplier is used

8.

Bob, the local car dealer, is reducing all his cars by 15%. Find the reduced price of a Nissan which originally cost £690.

£580.60

£585.50

£586.50

£590.90

Another way is to use a multiplier. A 15% decrease is a mutliplier of 100 - 15 = 85%. 85% = 0.85 (just divide 85 by 100). £690 x 0.85 = £586.50

9.

The local football team are playing badly and the number of season tickets sold reduces by 17% from last year. If 3,700 tickets were sold last year, how many were sold this year?

3,000

3,071

3,091

3,099

To find 17% multiply by 0.17

10.

To find the result of increasing a number by 13% you multiply it by .......

1.13

1.3

1.31

87

If something is increased by x% then its original value will have to be multiplied by (1 + x) to find the new value. The 1 represents 100% and the x represents the percentage increase: in this case (1 + x) = (1 + 0.13) = 1.13. If there was a decrease of x%, then you would have to multiply the original value by (1 - x)

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