Question

Helium is pumped into a spherical balloon at a constant rate of 4ft^3/second. How fast is the radius increasing after 1 minute? After 2 minutes? Is there any time at which the radius is increasing at a rate of 100 ft/sec? Explain ....I have set up dA/dt=2 x pi x r x dr/dt. I have dr/dt=4ft^3/sec and dA/dt=60sec and 120sec. I'm not sure where to go from here and not sure dr/dt and dA/dt's are correct.