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

The answer is that the rate of change is $50 per 1 monthhope this ; )