Area of a square natural lake is 50 sq. kms. A driver wishing to cross the lake diagonally, will have to swim a distance of :

Answer: Option D

Let the length of the diagonal be x km
Then,
$$\begin{array}{rl}& \Rightarrow \frac{1}{2}{x}^2=50\\ & \Rightarrow x^2=100\\ & \Rightarrow x=10\text{km}\\ & \Rightarrow x=\left(\frac{10}{1.6}\right)\text{miles}\\ & \Rightarrow x=6.25\text{ miles}\\ & \left[\because 1\text{ mile}=\text{1.609 km}\right]\end{array}$$