Question

Find the volume of the iron used to make a hollow cylinder of height 9 cm and whose internal and external radii are 21 cm and 28 cm respectively.

• A. $9720c{m}^3$
• B. $9602c{m}^3$
• C. $9700c{m}^3$
• D. $9702c{m}^3$

Answer 1

The correct option is D $9702c{m}^3$

Let r, R and h be the internal radius, external radius and height of the hollow
cylinder respectively.
Given that, r =21 cm, R = 28 cm, h = 9 cm
Volume of hollow cylinder $=\pi \left(R^2-r^2\right)h{\mathrm{cm}}^3=\frac{22}{7}\times \left({28}^2-{21}^2\right)\times 9=\frac{22}{7}\times \left(28+21\right)\left(28-21\right)\times 9=9702{\mathrm{cm}}^3$