Need help with this formula. The tournament can have up to 20 players. All 20 are pointed as follows:

1st: 85 pts
2nd: 65 pts
3rd: 55 pts
4th: 45 pts
5th: 40 pts
6th: 35 pts
7th: 30 pts
8th: 25 pts
9th: 20 pts
10th: 15 pts
11th: 10 pts
12th: 9 pts
13th: 8 pts
14th: 7 pts
15th: 6 pts
16th: 5 pts
17th: 4 pts
18th: 3 pts
19th: 2 pts
20th: 1 pt

What formula can I use? I do not know anything about formulas. Can anyone help me?

Thank you very much!