a) $$\frac{{3}}{{2}}$$ hours
b) 7 hours
c) $$\frac{{29}}{{4}}$$ hours
d) $$\frac{{7}}{{2}}$$ hours

Answer: d
Explanation: Let the speeds of boat and stream was $$s$$ and $$v$$ km/hr respectively
Actual Speed Downstream = $$\left(s + v\right)$$  km/hr
Actual Speed upstream = $$\left(s – v\right)$$  km/hr
$$\eqalign{ & \frac{d}{{s + v}} + \frac{d}{{s – v}} = 5\,{\text{hr}}{\text{.}}\,15\,{\text{min}}{\text{.}} \cr & \Rightarrow \frac{d}{{s + v}} + \frac{d}{{s – v}} = \frac{{21}}{4}\,.\,….\left( 1 \right) \cr & {\text{and}} \cr & \frac{{2d}}{{s – v}} = 7 \cr & \Rightarrow \frac{d}{{s – v}} = \frac{7}{2}\,…..\left( 2 \right) \cr & {\text{By equation }}\left( 1 \right) – \left( 2 \right), \cr & \frac{d}{{s + v}} = \frac{{21}}{4} – \frac{7}{2} \cr & \Rightarrow \frac{d}{{s + v}} = \frac{{21 – 14}}{4} \cr & \Rightarrow \frac{d}{{s + v}} = \frac{7}{4} \cr & \Rightarrow \frac{{2d}}{{s + v}} = \frac{7}{2} \cr & \cr} $$
Hence, he takes $$\frac{{7}}{{2}}$$ hours to row 2d km distance downstream