I need a formula that will tell me the following:

I have specific number ranges that equal a locked value. I want to be able to put a (any) number in a cell, and it automatically put the value in another cell.
i.e.:

720 (user entered) | 18 (autopop)

because of the following rules:

700-850 = 18
850-970 = 20
etc.....

(I can enter this info on another sheet if needed.)