The external and internal diameters of a hemispherical bowl are 10 cm and 8 cm respectively. What is the total surface area of the bowl ?

Answer: Option B

Internal radius, r = 4 cm
External radius, R = 5 cm
Total surface area :
$$\begin{array}{rl}& =2\pi R^2+2\pi r^2+\pi \left(R^2-r^2\right)\\ & =3\pi R^2+\pi r^2\\ & =\left[\pi \left(3\times 25+16\right)\right]\text{ c}{\text{m}}^2\\ & =\left(\frac{22}{7}\times 91\right)\text{c}{\text{m}}^2\\ & =286\text{ c}{\text{m}}^2\end{array}$$