This activity is about multiplication and division by tenths.

 

Work out the following problems mentally.

 

Example 1 

0.9 × 0.9

9 × 9 = 81

As we've multiplied both 0.9's by 10 we've multiplied by a total of 100. This means we will need to divide our answer by 100.

So, 0.9 × 0.9 = 0.81

 

Example 2 

0.9 × 9,000

9 × 9,000 = 81,000

As we've multiplied 0.9 by 10 to get 9 we will need to divide our answer by 10.

So, 0.9 × 9,000 = 8,100

 

One of the easiest ways to multiply using decimals is to remove the decimal point, do the multiplication and then put the decimal point back, making sure that you have the same number of decimal places in the answer as in the original calculation.

 

Example 3 

8.1 ÷ 9

81 ÷ 9 = 9

As we've multiplied 8.1 by 10 to get 81 we will need to divide our answer by 10.

So, 8.1 ÷ 9 = 0.9

 

Example 4

8.1 ÷ 0.9

81 ÷ 9 = 9

As we've changed the two values using the same proportion, we don't need to do anything to the answer we get.

 

Let's have a go at some questions now.