Unit 6 Section 3 : Multiplying with Decimals

In this section we will extend our multiplication to include decimals.

It helps to think of any number which includes a decimal part as a whole number which has been adjusted by dividing by 10, 100, 1000 etc.
For example: 3.5 = 35 10
0.8 = 8 10
4.25 = 425 100
Try and work out how you would rewrite the following decimal numbers as whole numbers divided by 10, 100 or 1000. Once you think you know the answer to each one, click on the button marked Click on this button below to see the correct answer to see whether you are correct.
(a) 4.7
(b) 0.3
(c) 54.3

Now we can think about questions which have decimal parts in a different way.
Look carefully at the examples below - each stage is explained in square brackets.

Example 1 :3.5 19
=35 10 19[replace the 3.5 by 35 10]
=35 19 10[we can 10 and 19 in either order]
=665 10[35 19 = 665 using one of the methods in section 6.2]
=66.5
Example 2 :9.2 0.8
=92 10 8 10[replace 9.2 by 92 10 and replace 0.8 by 8 10]
=92 8 10 10[we can 10 and 8 in either order]
=736 10 10[92 8 = 736 using one of the methods in section 6.2]
=7.36

Practice Question
Work out the answer to this question on paper and then click on Click on this button below to see the correct answer to see whether you are correct.

What is 24 15.3?

 

Exercises

Work out the answers to the questions below and fill in the boxes. Click on the Click this button to see if you are correct button to find out whether you have answered correctly. If you are right then will appear and you should move on to the next question. If appears then your answer is wrong. Click on to clear your original answer and have another go. If you can't work out the right answer then click on Click on this button to see the correct answer to see the answer.

Question 1
Find the answers to the multiplications involving decimals below (but not using a calculator).

(a)3 0.8
(b)9 1.2
(c)5 0.7
(d)3 2.6
(e)6 1.5
(f)8 7.9
(g)2.1 3.2
(h)5.6 7.2
(i)8.4 2.1
(j)9.2 1.8
(k)1.2 6.2
(l)15 7.3
(m)22 9.4
(n)7.1 62
(o)74 5.3


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Produced by A.J.Reynolds January 2001