A Math Problem

this was my math problem for homework...check it out and solve please

A sphere is dropped into an atmospheric pressure zone. As the sphere drifts down, its diameter decreases by .09 inches per two seconds. Its initial volume is 50.54. Make a function solving for t (time) in terms of circumference (basically make the equation so C(t) =)

This is the 1 problem I could not understand...good luck

EDIT: in case you did not know, the volume of a sphere = 4/3(pi)(r^3)