Question

A lead pencil is in the shape of a cylinder. The pencil is $21cm$ long with radius $0.4cm$ and its lead is of radius $0.1cm$. What is the volume of wood in the pencil?

• A. $9.9c{m}^3$
• B. $8.9c{m}^3$
• C. $7.8c{m}^3$
• D. $8.5c{m}^3$

Answer 1

The correct option is A $9.9c{m}^3$

Volume of wood $=$ Volume of pencil $-$ Volume of lead

$=\pi (0.4{)}^2\times 21-\pi \times (0.1{)}^2\times 21$

$=\frac{22}{7}\times \left[\right(0.4{)}^2-\left(0.1{)}^2\right]\times 21$

$=\frac{22}{7}\times [0.16-0.01]\times 21$

$=\frac{22}{7}\times 0.15\times 21=9.9c{m}^3$