How to multiply decimals by whole numbers and by other decimals Multiplying Decimals How to multiply decimals by whole numbers and by other decimals

A reminder about multiplication If you have large numbers to multiply, one of the methods you could use to work them out is column multiplication. e.g. 1344 x 24 = 1344 x 24 5376 + 26880 32256 or 863 x 159 = 863 x 159 7767 + 43150 86300 137217

Multiplying a decimal by a whole number If you’re multiplying a decimal by a whole number, the easiest way to set it out is using the column method. Let’s imagine we are working out 153.7 x 9. Set it out in column method: 153.7 x 9 And it almost looks like you’re multiplying with whole numbers: 1537 x Now work out the sum as normal, but don’t forget to add in the decimal point, so that your answer has the same number of decimal places as your original number. 153.7 x 1 decimal place 1383.3 1 decimal place

Multiplying two decimals Multiplying two decimal numbers may seem more difficult, but as long as you follow these steps, it is relatively simple. Imagine you have to multiply 3.5 x 8.6. First set it out in column multiplication: 8.6 x 3.5 Now, multiply them as if they were whole numbers. 86 x 35 430 + 2580 3010 You have your answer, but where do the decimal points go. You might think that seeing as the numbers you’ve multiplied have one decimal place, so should your answer. But wait! Think about what you’re multiplying. If you round both original numbers, you get 9 x 4 = 36. Quite a long way from 301.0. So, what you need to do is add together the amount of decimal places you have in the question. In this case, 1 decimal place (8.6) plus 1 decimal place (3.5), making 2 decimal places altogether. This is how many decimal places your answer needs. 8.6 x (1 decimal place) 3.5 (1 decimal place) 30.10 (1 + 1 = 2 decimal places)

Let’s have a go with some different decimals: 0.037 x 0.08 0.037 x 0.08 37 x 8 296 0.037 x (3 decimal places) 0.08 (2 decimal places) 0.00296 (3 + 2 = 5 decimal places)

In Summary So when you’re multiplying a decimal number by another decimal, just remember to follow these steps, and you’ll have no problem: Set it out using column multiplication. Work it out as if they were whole numbers. Work out how many decimal places there are altogether. Place your decimal point in the answer according to the total number of decimal places.

Your Turn Now it’s your turn. Have a go at these and don’t forget to follow the steps. 345.2 x 6.7 0.023 x 0.18 21.91 x 42.75