A person can row a boat d km upstream and the same distance downstream in 5 hours 15 minutes. Also, he can row the boat 2d km upstream in 7 hours. How long will it take to row the same distance 2d km downstream?

Answer: Option D

Let the speeds of boat and stream was $$s$$ and $$v$$ km/hr respectively
Then, Actual Speed Downstream = $$\left(s+v\right)$$  km/hr
Actual Speed upstream = $$\left(s-v\right)$$  km/hr
According to question,
$$\begin{array}{rl}& \frac{d}{s+v}+\frac{d}{s-v}=5\text{hr}\text{.}15\text{min}\text{.}\\ & \Rightarrow \frac{d}{s+v}+\frac{d}{s-v}=\frac{21}{4}......\left(1\right)\\ & \text{and}\\ & \frac{2d}{s-v}=7\\ & \Rightarrow \frac{d}{s-v}=\frac{7}{2}......\left(2\right)\\ & \text{By equation }\left(1\right)-\left(2\right),\\ & \frac{d}{s+v}=\frac{21}{4}-\frac{7}{2}\\ & \Rightarrow \frac{d}{s+v}=\frac{21-14}{4}\\ & \Rightarrow \frac{d}{s+v}=\frac{7}{4}\\ & \Rightarrow \frac{2d}{s+v}=\frac{7}{2}\\ & \end{array}$$
Hence, he takes $$\frac{7}{2}$$ hours to row 2d km distance downstream