A cylinder has height 4. Each of its circular faces has a circumference of $10 \pi$. What is the volume of the cylinder?
Suppose that the cylinder has radius $r$.

Since the circumference of its circular faces is $10 \pi$, then $2 \pi r=10 \pi$ or $r=5$.

Therefore, the volume of the cylinder is $\pi r^{2} h=\pi \cdot 5^{2} \cdot 4=100 \pi$.

ANSWER: $100 \pi$