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 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 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 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 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

You have now completed Unit 6 Section 3
 Your overall score for this section is Correct Answers You answered questions correctly out of the questions in this section. Incorrect Answers There were questions where you used the Tell Me button. There were questions with wrong answers. There were questions you didn't attempt.
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