Posted by Mel on .
A spherical party balloon is being inflated with helium pumped in at a rate of 12 cubic feet per minute. How fast is the radius growing at the instant when the radius has reached 1 ft?

mathcalculus 
Reiny,
V = (4/3)πr^3
dV/dt = 4π r^2 dr/dt
given : dV/dt = 12 , and r = 1
12 = 4π (1) dr/dt
dr/dt = 3/π ft/min 
mathcalculus 
Mel,
Thank you! I initially thought the given rate of 12 cubic feet per minute was dr/dt.

mathcalculus 
Reiny,
The units are usually your giveaway.
it says 12 cubic feet/min which is volume/time , so dV/dt
This will work in most cases. 
mathcalculus 
12,
12