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, then one of the easiest ways to work them out is using column multiplication. e.g. 1344 x 24 = 32256 863 x 159 = 137217 1344 x 863 x 24 159 5376 + 7767 + 26880 43150 32256 86300 137217

Multiplying a decimal by a whole number When you have to multiply a decimal by a whole number, using column multiplication makes it easy. Imagine you have 153.7 x 9. If you set it out using column multiplication, then it looks like you’re multiplying a whole number: 153.7 x 9 1383.3 When you’ve got your answer, you can work out where you put the decimal point by checking how many decimal places the original decimal number had. 153.7 has 1 decimal place, so we need to put the decimal point in our answer so it has one decimal place.

Multiplying two decimals!!!! Ok, lets up the stakes! Multiplying a decimal number by another decimal might at first seem difficult, and yes it’s easy to make mistakes. But if you follow some simple steps, it’s actually a doddle, no matter the size of the decimal! 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. WAIT!! Think about your 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) and 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. 037 x 0. 08 0 Let’s have a go with some different decimals. 0.037 x 0.08 0.037 x 37 x 0.037 x (3 decimal places) 0.08 8 0.08 (2 decimal places) 296 0.00296 (3 + 2 = 5 decimal places) 2.04 x 1.95 2.04 x 204 x 2.04 x (2 decimal places) 1.95 195 1.95 (2 decimal places) 1020 + 3.9780 (2 + 2 = 4 decimal places) 18360 20400 39780

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