What's the equation?

I have some data, but I can’t seem to figure out the function eqation that matches the data.
There are 2 variables, R and D.
here is a chart:
R | D


5 | 50
15 | 50
45 | 45
50 | 41

so in other words, when R is 5 then D is 50. But… given that data I can’t figure out the equation f®=D.

Just if your’e wondering, this is for a flash game I am making; it has to do with the _rotation of something and the y velocity of something else…