A well of radius 3 m is dug 7 m deep. The earth taken out is spread evenly to make a ring-shaped…

Step 1. The earth. $\pi \times 3^2 \times 7 = 63\pi\ \text{m}^3$.

Step 2. The ring. The outer radius is $3\ \text{m} + 4\ \text{m} = 7\ \text{m}$. The area is $\pi(7^2 - 3^2)\ \text{m}^2 = 40\pi\ \text{m}^2$.

Step 3. The height. $\dfrac{63\pi\ \text{m}^3}{40\pi\ \text{m}^2} = 1.575\ \text{m}$.