Page 40 - Mathematics Coursebook

3.5 Adding and subtracting decimals

) Exercise 3.5

1 Use a mental method to work out the answers to these. Break the numbers into their whole-number
a 3.5 + 4.2 b 4.6 + 3.7 c 12.7 + 4.5 parts and their decimal parts.
d 4.9 − 1.5 e 12.6 − 5.2 f 14.6 − 6.8

2 Use a different mental method to work out the answers to these.
a 4.9 + 7.3 b 9.6 + 8.9 c 22.8 + 3.3 Use the method of rounding
d 5.4 − 1.9 e 14.9 − 4.4 f 21.1 − 6.7 one of the numbers to a
whole number.
3 Alicia works out 7.5 + 4.8 in her head like this. She says:

‘If I round 4.8 up to 5, I can change 7.5 + 4.8 to 7.5 + 5, which
equals 12.5. Then I must add the extra 0.2, which gives me 12.7.’

Is Alicia correct? Explain your answer.
Is Alicia correct? Explain your answer.
4 Use a written method to work these out.
a 5.79 + 4.15 b 25.81 + 58.4 c 8.76 − 4.14 d 38.9 − 19.78
5 At the cinema, Priya spends $4.75 on a ticket, $1.75 on sweets and $0.85 on a drink.
a How much does she spend altogether?
b Priya pays with a $10 bill.
How much change does she receive?

6 Jed is a plumber.
He has four lengths of pipe that measure
1.8 m, 3.5 m, 2.45 m and 0.85 m.
a What is the total length of the four pipes?
b Jed needs 10 m of pipe altogether.
How much more pipe does he need to buy?
7 Moira went on a diet. She recorded her weight at the start and end of every month.
Here are her records for June and July.
Here are her records for June and July.

Weight (kg) Weight (kg)
Start of June 95.45 End of June 91.92
Start of July 91.92 End of July 88.35

a During which month, June or July, did Moira lose more weight?
b At the start of August Moira weighed 88.35 kg. During August she lost 1.82 kg.
How much did Moira weigh at the end of August?
8 Work out the missing digits in these calculations.

a 2 . 1 b 6 . 6 4
+ 2 . 3 − 9 . 5
6 9 . 1 6 3 8 . 6

3 Place value, ordering and rounding 39