Multiplication with decimal numbers

There's a fruit sale at the market. Apples cost 20 kronor per kilogram and you want to buy 0,9 kilograms of apples. How much do you have to pay? To be able to answer this question, it is helpful to know several tricks. 1 times 10 equals 10.

1.5 times 10 equals 15. When you multiply by a number greater than 1 the product is larger. 0.5 times 10 equals 5.0 When you multiply by a number less than 1 the product is smaller. We can use this when we multiply by other decimals as well. When you multiply decimals that contain zeros, you can simplify them by making one factor 10 or 100 times smaller and the other factor 10 or 100 times larger without changing the product.

Example: 0.8 times 0.06 We make the first factor 10 times larger and the second factor 10 times smaller. 0.8 times 10 equals 8 and 0.06 divided by 10 equals 0.006 which gives 8 times 0.006 equals 0.048 Another example: 0.8 times 600 Here, we make the first factor 100 times larger and the second factor 100 times smaller. 0.8 times 100 equals 80 and 600 divided by 100 equals 6 which gives 80 times 6 equals 480 We simplified the problem by making one factor an integer, which means that we have a simpler calculation. So, what do your apples cost? 0.9 kilograms of apples for 20 kronor per kilo.

To calculate the total cost you have to multiply 0,9 by 20. Now you can use the tricks you just learned. Make the first factor 10 times larger and the other factor 10 times smaller. 0,90 times 10 equals 9 and 20 divided by 10 equals 2 which gives 9 times 2 equals 18 The answer is reasonable, since 1 times 20 equals 20, so 0,9 times 20 must be slightly less than 20. The answer is always smaller when you multiply by a number that is less than 1.

Your apples cost 18 kronor.