Let V = volume (in ft 3 ) of the balloon in t minutes
      r = radius (in ft) of the balloon after t minutes
Given:  dV/dt = 4.5  Find:  dr/dt when r = 2
V = (4/3)πr3
So, dV/dt = 4πr2(dr/dt) 
     4.5 = 4π(2)2(dr/dt)
     dr/dt = 9/(32π) ft/min
 
     
             
                    