Question

How many pairs of whole numbers have a sum of 40

Answer 1

So,

We start with 0 + 40.
1. 0 + 40
2. 1 + 39
3. 2 + 38
4. 3 + 37
...
...
...
19. 18 + 22
20. 19 + 21
21. 20 + 20

Tada!  There are 21 pairs of whole numbers that have a sum of 40.