Multiplying decimals

Multiplying decimals works the same way as multiplying whole numbers.

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When multiplying decimals, add up the number of digits after the decimal points in the question. This number tells you the number of decimal places you should have in your answer.

So, if the question includes one decimal place in total, 3.2 \times 6, then the answer must include one decimal place, 19.2. If the question has two decimal places in total, 4.2 \times 2.8, then the answer must have two decimal places, 11.76.

Example

What is 3.72 \times 2.3?

First, do the calculation with whole numbers, so work out 372 \times 23.

Example of long multiplication

Note that there are three decimal places in the calculation (3.72, 2.3), so there needs to be three decimal places in the answer. The answer is therefore 8.556.

Question

What is 5.2 \times 8?

First, work out 52 \times 8.

Example of long multiplication

Next, note the number of decimals in the question - just one - and make sure that there is an equal number of decimal places in the answer.

The answer is therefore: 41.6

Remember, if your answer has zeros in the decimal places then you must count these too.

Question

What is 3.4 \times 5.5?

First, work out 34 \times 55.

Example of long multiplication (34 * 5)

As before, note the number of decimal places in the question - two - and make sure that there is an equal number of decimal places in the answer.

The answer is therefore 18.70 which would be written as: 18.7