No he would get back $4.56. Start with $20 -13.59-1.85=4.56
You can use calculus ( related rates). Given the rate os change of the radius for example you can find the rate of change of volume using differential calculus.
(1,4), (-1,3) 3-4/-1-1=-1/-2. -1/-2= 1/2 so 1/2 would be the slope
