A spherical ball of salt is dissolving in water in such a manner that the rate of decrease of volume at any instant is proportional to the surface. Prove that the radius is decreasing at a constant rate.

Explanation

To solve the problem, we need to show that the radius \( r \) of a spherical ball of salt is decreasing at a constant rate as it dissolves in water, given that the rate of decrease of volume is proportional to the surface area of the sphere.

### Step-by-Step Solution:

1. **Understand the Volume and Surface Area of a Sphere**:
The volume \( V \) of a sphere is given by the formula:
\[
V = \frac{4}{3} \pi r^3
...