Subtraction

You pay with a ten-euro note and want to know how much change you get. Or you check how many pages of your book are left. Both are subtraction. It answers two questions that come up all the time: what is left over, and how big is the difference? Here you'll learn how to answer both of them easily.

Jumping backwards

Subtracting means taking away. On the number line you jump backwards: for 9 − 4 you start at 9 and take 4 steps to the left, landing at 5. But subtraction also answers a second question: how big is the difference between two numbers? The difference between 41 and 38 is 3, whether you jump down from 41 or count up from 38.

Addition in reverse

Subtraction undoes addition: if 8 + 6 = 14, then 14 − 6 = 8 as well. That gives you a built in check. After working out 14 − 6 = 8, test it with 8 + 6: if you get 14, your answer is right. You can check yourself any time, no calculator needed.

The tens trick backwards and subtracting by place

You know the tens trick from adding, and it works backwards too: for 13 − 5, jump back 3 to the round 10 first, then the remaining 2, landing at 8. With bigger numbers, subtract place by place, tens first and ones after: for 84 − 46, work out 84 − 40 = 44 and then 44 − 6 = 38. Round stopovers keep every subtraction manageable.