related rates of a snowball

Question

A spherical snowball is melting in such a way that its radius is decreasing at rate of 0.4 cm/min. At what rate is the volume of the snowball decreasing when the radius is 16 cm.

(Note the answer is a positive number).

Please help!

Answer 1

The volume of a sphere is (4/3)pir^3 take the derivative of both sides

dV= (4/3)pi ( 3r^2) dr You are told dr=0.4 and you want dV when r=16

dV= 4pi(16)^2(.4)= 1687cm^3/min