What will be the area of a semi-circle whose perimeter is 36 cm ?

Answer: Option E

Given :
$$\begin{array}{rl}& \pi R+2R=36\\ & \iff \left(\pi +2\right)R=36\\ & \iff R=\frac{36}{\left(\frac{22}{7}+2\right)}cm\\ & \iff R=7cm\end{array}$$
Required area :
$$\begin{array}{rl}& =\frac{\pi R^2}{2}\\ & =\left(\frac{22}{7}\times \frac{7\times 7}{2}\right)c{m}^2\\ & =77c{m}^2\end{array}$$