As a balloon in the shape of a sphere is being blown up, the radius is increasing 1/pi inches per second. At what rate is the volume increasing when the radius is 1 inch?

Question

I know that the volume of a sphere = 4/3(pi)r^3

I don't know what to do next.

Figure dV/dt

dV/dr *dr/dt= dV/dt

you know the expression for dv/dr (take the derivative of V(r), and you are given dr/dt as 1/pi per sec.