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