Page 41 - Mathematics Coursebook

3.6 Multiplying decimals

3.6 Multiplying decimals

When you multiply a decimal number, you must
remember the decimal place-value table. Units • Tenths Hundredths Thousandths

Follow these steps when you multiply a decimal 1 s 1 1 1
by a single-digit number. 10 100 1000
t At 'rst, ignore the decimal point and work out the multiplication.
t Finally, put the decimal point in the answer. !ere must be the same number
of digits a$er the decimal point in the answer as there were in the question.

Worked example 3.6
a Work these out mentally. i 0.2 × 4 ii 0.6 × 2
b Use a written method to work out 4 × 2.16.

a i 2 × 4 = 8 Ignore the decimal point and work out 2 × 4.
0.2 × 4 = 0.8 Put the decimal point back into the answer. There’s 1 digit after the decimal point
in the question, so there must be 1 digit after the decimal point in the answer.
ii 6 × 2 = 12 Ignore the decimal point and work out 6 × 2.
0.6 × 2 = 1.2 Put the decimal point back into the answer. There’s 1 digit after the decimal point
in the question, so there must be 1 digit after the decimal point in the answer.

b 2 1 6 Ignore the decimal point and work out 216 × 4.
× 4
8 6 4
Put the decimal point back into the answer. There are 2 digits after the
2
decimal point in the question, so there must be 2 digits after the decimal
4 × 2.16 = 8.64 point in the answer.

) Exercise 3.6

1 Use a mental method to work these out.
a 0.1 × 8 b 0.3 × 3 c 0.5 × 5 d 0.7 × 6 e 0.9 × 2

2 Use a written method to work these out.
a 5 × 2.7 b 8 × 3.6 c 3 × 9.8 d 3.15 × 2
3 Use the numbers from the box to complete these calculations. 18.3 2 36.8
You can only use each number once. You should have no 0.6 6.1 7 0.7
numbers left at the end.
a 0.1 × 6 = b 0.4 × = 2.8 c × 5 = 3.5
d 4.3 × = 8.6 e 9.2 × 4 = f × 3 =

4 Anders and Jake both work out the answer to 0.8 × 5.
Anders says: ‘The answer is 4.0.’ Jake says: ‘The answer is 4.’
Are they both correct? Explain your answer.

40 3 Place value, ordering and rounding