Calculus word problem

Question

Air is being filled in a ballon with 28 cubic feet per min . At what rate is the radius changing when the radius is 3 ft

I got 7/9pi 

Answer 1

You are correct!

 

V= 4/3*pi*r^3

 

dV/dt = 4*pi*r^2*dr/dt

 

   28 = 4* pi * 3^2 * dr/dt

 

dr/dt= 7/(9*pi)