Page 79 - MAT KS3 Y8 Cambridge CheckPoint

7.5 Finding fractions of a quantity

7.5 Finding fractions of a quantity

To work out a fraction of a quantity, divide the quantity by the You can use the multiplication facts
denominator of the fraction, then multiply your answer that you know to work out simple
by the numerator. fractions of quantities mentally.

Worked example 7.5a
3
Work out of 30 kg.
5
1
30 ÷ 5 = 6 First, find of 30 kg by dividing 30 by 5.
5
3
6 × 3 = 18 kg Then multiply your answer of 6 by 3 to find of 30 kg.
5
Remember to include the units (kg) in your answer.
When you find a fraction of a quantity, you do not always get a whole-number answer. You know this is
going to happen when the denominator doesn’t divide into the quantity exactly.
When this happens, the best method is to multiply the quantity by the numerator first. Then divide the
answer by the denominator and write your final answer as a mixed number.

Worked example 7.5b
2
Work out of 20 km.
3
2 × 20 = 40 3 does not divide exactly into 20, so multiply 20 by 2 instead.
1 1
40 ÷ 3 = 13 km Now divide 40 by 3. This gives 13, with a remainder of 1, so the answer is 13 .
3
3
Remember to include the units (km) in your answer.
✦ Exercise 7.5

1 Work these out mentally.
a 3 of $12 b 2 of 10 m c 4 of 21 kg d 5 of 40 cm e 6 of 33 ml
4 5 7 8 11
2 Work out these fractions of quantities. Give each answer as a mixed number.
3 2 3 4 5
a of 16 kg b of 23 t c of $33 d of 47 mg e of 25 mm
5 3 8 9 6
3 Wyn has six blue question cards and five yellow answer cards.

14 m 10 m 4 7
9 of 30 m 12 of 24 m

1
13 m
2 3 5 5
11 m 2 of 18 m of 14 m
3 12 m 9
3 6
2 3
a Match each blue card with the correct yellow card. 3 of 19 m 14 of 56 m
b What is the answer on the missing yellow card?

7 Fractions 77