The constant of proportionality is k > 0, V is the volume, S is the surface area. Remember that you must use the prime notation for derivatives, not the Leibniz notation, in all of your answers.
A raindrop evaporates at a rate that is proportional to the surface area, write this as a differential equation: $= 0$.

Since $V = \frac{4}{3}\pi r^3$ we have by the chain rule that $\frac{dV}{dt} = V^\prime =$ . Substituting this into the original differential equation, along with the equation $S = 4\pi r^2$, and solving for $r^\prime$ we find that
$\frac{dr}{dt} =$ so that $r =$
Therefore if radius is initially $3$ then $V =$ .