Let’s count them. Harry has 3 apples left. So, one less than 4 is 3. One more than 6 is 7. You can also use a number line to show this. On a number line you can see that 1 more than 6 is 7.

When you count to 20, you start at 1 (or zero) and count on until you reach 20. Each number is one more than the last. When you count back from 20, you start at 20 and count back until you reach 1 ...