related rates problemcalculus
posted by james .
A spherical balloon is being filled with air in such a way that its radius is increasing at a rate of 2 centimeters per second. At what rate is the volume of the balloon changing at the instant when its surface has an area of 4 pi square centimeters?

The balloon is spherical, so
V = (4/3)(pi)r^3
SA = 4(pi)r^2
Differentiate both with respect to time.
dV/dt = 4(pi)r^2(dr/dt)
dSA/dt = 8(pi)r(dr/dt)
We're given the rate of change in SA and need to find the rate of change in volume. Let's write an equation for it.
(dV/dt)/r = 4(pi)r(dr/dt)
(dSA/dt)/2 = 4(pi)r(dr/dt)
(dV/dt)/r = (dSA/dt)/2
(dV/dt) = (r/2)(dSA/dt) 
Edit: actually we're given dr/dt and SA.
Since dV/dt = 4(pi)r^2(dr/dt) and SA = 4(pi)r^2, dV/dt = SA(dr/dt).