Need algorithm to turn percent into cost per acquisition average

Question

Can anyone find the algorithm so I can take a % and calculate the $ value if the following statements are true:

20%=$90
30%=$60
40%=$45
50%=$36
60%=$30
100%=$18

For example: what is the dollar value of 19.5%???  

 

Answer 1

plotting the dollar amount (y) vs percent (x) yields something like a 1/x curve. Then plotting y vs 1/x gives a straight line, for which you can find the slope. In fact:

 

$y=(1800%)/(x%)

 

so for x=19.5%, have y=1800/19.5=$92.31.