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

Explanation

To form a differential equation involving the rate of change of the radius of a raindrop, we can follow these steps:

### Step 1: Understand the relationship between evaporation and surface area
We know that the rate of evaporation of a raindrop is proportional to its surface area. Let's denote:
- \( V \) as the volume of the raindrop.
- \( r \) as the radius of the raindrop.
- \( A \) as the surface area of the raindrop.

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