Year 7 Interactive Maths - Second Edition

Multiplication of a Decimal Number by another Decimal Number

• ignore the decimal points and multiply the digits;
• count the total number of decimal places in both decimal numbers being multiplied; and
• place a decimal point in the answer so that it has the same number of decimal places as the total number of decimal places in the two numbers being multiplied

Example 23

Calculate 0.8 × 0.9.

Solution:

Note:

• There is 1 decimal place in 0.8 and 1 decimal place in 0.9 and thus, 2 decimal places in the answer.
• First ignore the decimal points and multiply 8 by 9.  Then include the 2 decimal places in the answer.

Example 24

Calculate 0.78 × 0.5.

Solution:

Note:

• There are 2 decimal places in 0.78 and 1 decimal place in 0.5 and thus, 3 decimal places in the answer.
• First ignore the decimal points and multiply 78 by 5.  Then include the 3 decimal places in the answer.

Example 25

Calculate 3.24 × 0.67.

Solution:

Note:

• There are 2 decimal places in 3.24 and 2 decimal places in 0.67 and thus, 4 decimal places in the answer.
• First ignore the decimal points and multiply 324 by 67.  Then include the 4 decimal places in the answer.