Page 83 - Mathematics Coursebook

7.6 Finding fractions of a quantity

7.6 Finding fractions of a quantity

You work out a unit fraction of a quantity by dividing the quantity by the denominator of the fraction.
1
1
Example: To work out of 18 cm, divide 18 cm by 3. So of 18 cm = 18 ÷ 3 = 6 cm.
3 3
2
To work out a more complicated fraction such as , divide the quantity by the denominator and then
multiply your answer by the numerator. 3
2
Example: To work out of 18 kg, you divide 18 kg by 3, then multiply by 2.
3
2
18 ÷ 3 = 6, 6 × 2 = 12. So of 18 kg = 12 kg.
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Worked example 7.6
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Work these out. a 1 3 of 15 cm b of 20 kg If you can’t work out part c in your
5
head, use a written method or a
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c 7 × 105 calculator.

a 15 ÷ 3 = 5 cm Divide the quantity (15 cm) by the denominator (3).

b 20 ÷ 5 = 4 First of all fi nd 1 by dividing the quantity (20 kg) by the denominator (5).
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4 × 2 = 8 kg Then multiply the answer by 2 to fi nd 2 .
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c 105 ÷ 7 = 15 First of all fi nd 1 7 by dividing the number ‘Of’ and ‘×’ mean exactly the same
thing, so use the same method.
(105) by the denominator (7).
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15 × 4 = 60 Then multiply the answer by 4 to fi nd 7 . There are no units in this answer.

) Exercise 7.6

1 Work these out mentally.
a 1 of $8 b 1 of 18 km c 4 × 18 d 3 × 28
2 6 9 7
2 Use a written method or a calculator to work these out.
a 2 of $182 b 4 of 195 miles c 3 × 192 d 13 × 345
7 13 8 15

3 Which of these cards is different from the others? 28 × 4 27 × 2 26 × 9
Explain your answer. 7 3 13

4 At a rugby match in the Ellis Park Stadium in Johannesburg there
were 58 476 rugby fans.
7 of the fans were supporting South Africa. The rest were
12
supporting Australia.
How many fans were supporting Australia? How can you tell if your
answer is likely to be correct?

82 7 Fractions