Unit weight of iron
$\gamma = \dfrac{W}{V}$
$\gamma_{iron} = \dfrac{4}{\frac{4}{3}\pi (5^3)}$
$\gamma_{iron} = \dfrac{3}{125\pi} \, \text{ kg/cm}^3$
Radii of iron shell (R = external radius and r = internal radius)
$R = \frac{1}{2}(50)$
$R = 25 \, \text{ cm}$
$r = R - t = 25 - 5$
$r = 20 \, \text{ cm}$
Volume of iron shell
$V = \frac{4}{3}\pi (R^3 - r^3)$
$V = \frac{4}{3}\pi (25^3 - 20^3)$
$V = \dfrac{30\,500\pi}{3} \, \text{ cm}^3$
Weight of iron shell
$W = \gamma_{iron} V = \dfrac{3}{125\pi} \left( \dfrac{30\,500\pi}{3} \right)$
$W = 244 \, \text{ kg}$ answer