A cylindrical tube open at both ends is made of metal. The internal diameter of the tube is 11.2 cm and its length is 21 cm. The metal everywhere is 0.4 cm thick. The volume of the metal is :

Internal radius :
$$\eqalign{ & = \left( {\frac{{11.2}}{2}} \right)cm \cr & = 5.6\,cm \cr} $$
External radius :
$$\eqalign{ & = \left( {5.6 + 0.4} \right)cm \cr & = 6\,cm \cr} $$
Volume of metal :
$$\eqalign{ & = \left[ {\frac{{22}}{7} \times \left\{ {{{\left( 6 \right)}^2} - {{\left( {5.6} \right)}^2}} \right\} \times 21} \right]c{m^3} \cr & = \left( {66 \times 11.6 \times 0.4} \right)c{m^3} \cr & = 306.24\,c{m^3} \cr} $$