Respuesta :

We start with 0 + 40.
1. 0 + 40
2. 1 + 39
3. 2 + 38
4. 3 + 37
.....
keep going....
.....
19. 18 + 22
20. 19 + 21
21. 20 + 20 
There are 21 pairs of whole numbers that have a sum of 40.