How many pairs of whole numbers have a sum of 40

For the writer, there are 20

1. 1 + 39 = 40

2. 2 + 38 = 40

3. 3 + 37 = 40

4. 4 + 36 = 40

5. 5 + 35 = 40

6. 6 + 34 = 40

7. 7 + 33 = 40

8. 8 + 32 = 40

9. 9 + 31 = 40

10. 10 + 30 = 40

11. 11 + 29 = 40

12. 12 + 28 = 40

13. 13 + 27 = 40

14. 14 + 26 = 40

15. 15 + 25 = 40

16. 16 + 24 = 40

17. 17 + 23 = 40

18. 18 + 22 = 40

19. 19 + 21 = 40

20. 20 + 20 = 40