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3.4 Adding and subtracting decimals

3.4 Adding and subtracting decimals

When you are adding or subtracting decimal numbers, always set out the calculation in columns and
remember to keep the decimal points in line.

Worked example 3.4

Work these out. a 14.7 + 8.56 b 13.5 − 1.72

a 14 . 7 Start with the hundredths column: 0 + 6 = 6. The blank space means zero.
+ 8 . 5 6 Next add the tenths: 7 + 5 = 12, write down the 2 and carry the 1.
23 . 26 Now add the units: 4 + 8 + 1 = 13, write down the 3 and carry the 1.
1 1 Finally, add the tens: 1 + 1 = 2.
b 1 3. 5 0 Start by writing 13.5 as 13.50, then start subtracting with the hundredths column.
1
14
2
1. 7 2 You can’t subtract 0 − 2. ‘Borrow’ from the 5 and work out 10 − 2.
11 . 7 8 Now subtract the tenths. You can’t subtract 4 − 7. ‘Borrow’ from the 3 and work
out 14 − 7.
Now subtract the units, and finally the tens.

✦ Exercise 3.4

1 Work these out.
a 6.24 + 8.35 b 11.42 + 25.39 c 4.78 + 8.43 d 19.45 + 9.83
e 23.3 + 5.42 f 16.77 + 9.5 g 8.72 + 14.9 h 123.8 + 9.37
i 0.48 + 7.8 j 67.043 + 5.672 k 9.95 + 0.478 l 12.376 + 7.8
2 Work these out.
a 4.72 − 2.51 b 23.78 − 9.35 c 13.73 − 2.44 d 19.38 − 6.65
e 48.65 − 12.78 f 32.27 − 1.49 g 82.77 − 25.93 h 45.42 − 7.35
i 74.9 − 3.67 j 11.8 − 4.36 k 34.9 − 8.77 l 1.75 − 0.688

3 Part of Jake’s homework is on the right. Work out 35– 4.47
4 9 1
Use Jake’s method to work these out. 35 . 00
a 23 − 2.65 b 46 − 1.76 c 87 − 13.45 – 4 . 47
d 245 − 22.49 e 16 − 0.76 f 42 − 4.66 30 . 53
g 58 − 9.06 h 235 − 18.18

4 Nelson’s column in London, UK, is a monument that was built in memory of
Admiral Horatio Nelson, who died at the Battle of Trafalgar in 1805.
It has a base, 14.11 m high. On top of this is a column 47.55 m high. At the top,
there is a statue 5.18 m high.
What is the total height of Nelson’s column?

5 The table shows the results of a javelin competition, in metres.
Position Distance thrown (m)
Is the difference in the distances thrown by the athletes who
1 70.20
finished first and second larger than the difference in the
2 67.51
distances thrown by the athletes who finished second and
3 64.84
third? Show how you worked out your answer.
3 Place value, ordering and rounding 37