Multiplying a decimal

Pupils can struggle to set out calculations involving decimals, if they attempt to line the numbers up as they would for addition and subtraction. For example:

Pupils may use placeholders to fill in the gaps, often incorrectly, and can be confused about where to write the digits in their answer. Trying to align the numbers will lead to further problems when working with decimals in long multiplication.

Pupils can work out these calculations by ignoring place value when setting the calculation out and simply lining up the decimal point in the answer. However, it is important to note that this will not result in a correct answer if both numbers are decimals.

Here, the pupil included the decimal point as they worked through the calculation, but didn’t carry correctly across it. They need to understand that 6 x 4 gives them 24 tenths, which means the 2 should be carried across the decimal point (as it means two whole ones) with the 4 being written in the tenths place.

Making the link between known multiplication facts and multiplying by decimals can help pupils to realise that the digits are the same in both answers, but the place value in each is different. For example:
10 x 5 = 50
1 x 5 = 5
1 x 0.5 = 0.5
30 x 5 = 150
3 x 5 = 15
3 x 0.5 = 1.5
213 x 3 = 639
21.3 x 3 = 63.9
2.13 x 3 = 6.39
Click here to generate a worksheet to practise multiplying a decimal.