Multiplying two decimals (written)
You can work out the answer to any decimal multiplication without a calculator using normal written multiplication methods. It doesn't matter which you use, although long multiplication is the fastest.
As for more simple decimal calculations, work out the calculation using whole numbers. So for 3.4 x 2.8, you would work out 34 x 28 using whichever multiplication method you prefer. You should get the answer 958.
Now compare the calculation 34 x 28 to the calculation 3.4 x 2.8. Both 3.4 and 2.8 are 10 times smaller than 34 and 28, so the answer will be 100 times smaller. Divide 958 by 100 to get 9.58.
You can also use the decimal place counting trick to help. The number of decimal places in the question is the same as the number in the answer.
3.4 x 2.8 = 9.58
You should also estimate to check that your answer is about the right size. Both 3.4 and 2.8 round to 3 (nearest whole number. 3 x 3 = 9, so the answer 9.58 is reasonable for 3.4 x 2.8.
- BBC Bitesize: Notes and a couple of questions to try.
- IXL: Online questions with solutions; starts with more simple problems but gets more complicated quite quickly.
- Maths is Fun: Multiplying decimals animation.
- Mymaths: Interactive lesson on multiplying decimals. Test yourself with the online homework.
- Manga High: Online game/questions with solutions to check your answers.