JAMB 1988 · UME · Q44

A metal pipe 10 cm long has an external diameter of 12 cm and a thickness of 1 cm. Find the volume of the metal used in making the pipe.

Worked solution (try it first)

External radius: $12 \div 2 = 6$ cm.
The wall is 1 cm thick, so the internal radius is $6 - 1 = 5$ cm.
The metal is the ring between the circles times the length: $\pi(6^2 - 5^2) \times 10$ .
That is $\pi \times 11 \times 10 = 110\pi\text{ cm}^3$ , option B.