Page 42 - Cambridge Checkpoint Mathematics Coursebook 7_Slide 02

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 • 1 1 1
by a single-digit number. 10 100 1000
• At first, ignore the decimal point and work out the multiplication.
• Finally, put the decimal point in the answer. There must be the same number
of digits after 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.

F 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