Let the rate that the volume increases/decreases be represented by:
dV/dt = 4.9
Your goal is to solve for dr/dt (the rate at which the radius is increasing/decreasing).
Start with the general formula for the volume of a sphere:
V = (4/3)π r 3
Differentiate both sides with respect to t (time).
dV/dt = 4π r 2(dr/dt)
Plug in the information provided in the question, dV/dt = 4.9 and r = 1.5.
4.9 = 4π (1.5)2 (dr/dt)
Solve for the expression dr/dt, which represents the rate the radius is changing.
Ask me if you are unsure about anything in this explanation.