Assume that a raindrop evaporates at a rate proportional to its surface area. Form a differential equation involving the rate of change of the radius of the raindrop.

Let r be the radius of the raindrop, V be its volume and A be its surface area

Given

Negative because V decreases with an increase in t

is a proportionality constant

Now, we know that

So, we have,

Hence, the required differential equation is