JAMB 1990 · UME · Q42

Solid mensuration

A cylindrical metal pipe is 3 cm thick. If the internal radius of the pipe is 10 cm, find the volume of metal used in making 3 m of the pipe.

Worked solution (try it first)

Internal radius 10 cm, wall 3 cm, so the external radius is 13 cm.
The length is 3 m, which is 300 cm.
Area of the ring: $\pi(13^2 - 10^2) = \pi(169 - 100)$
$= 69\pi\text{ cm}^2$ .
Volume: $69\pi \times 300 = 20\,700\pi\text{ cm}^3$ , option D.

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