1. The problem statement, all variables and given/known data A spherical raindrop evaporates at a rate proportional to its surface area. Write a differential equation for the volume of the raindrop as a function of time. 2. Relevant equations 3. The attempt at a solution The answer is dV/dt = -kV^(2/3), for some k > 0. I don't really understand this answer. The question states the volume changes at a rate proportional to its surface area (A = 4*pi*r^2), but it seems from the answer it depends on volume V. Can someone help me understand how to get to this answer? And how do I know there is a constant k in there too? The problem never states it.. am I just supposed to know this somehow? Thanks!