How many pairs of whole numbers that have a sum of 40

1 + 39

2 + 38

3 + 37

4 + 36

5 + 35

6 + 34

7 + 33

8 + 32

9 + 31

10 + 30

11 + 29

12 + 28

13 + 27

14 + 26

15 + 25

16 + 24

17 + 23

18 + 22

19 + 21

20 + 20

Then it would start repeating itself the other way round, so the answer is 20 pairs