is the inverse of — it undoes it. If you add b and then take the same b away again, you are right back where you started.

So whenever you know an addition fact:

you can run it backwards to get two subtraction facts:

Watch how the marker hops right by b to add, then hops left by the same b to subtract — and lands exactly where it began. The left hops perfectly undo the right hops. Replay it: each time it uses fresh numbers.

Those three numbers a, b and c make a little team called a fact family. From one team you can write four true number sentences — two adding and two subtracting:

For example, the family 3, 4, 7 gives:

This gives you two superpowers. First, you can check a subtraction by adding back: if c - b = a is right, then a + b should bring you back to c.

Second, you can subtract by thinking addition. To work out 12 - 5, don't count back — instead ask "5 plus what makes 12?". Since 5 + 7 = 12, you know 12 - 5 = 7. The missing number is the answer.

Khan Academy explains how addition and subtraction relate here: