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?

Explanation:-

Let the speeds of boat and stream was S and V km/hr respectively.
Then, Actual Speed Downstream = (S + V) km/hr.
Actual Speed upstream = (S - V) km/hr.
Given,
$$\eqalign{ & \left\{ {\frac{d}{{\left( {S + V} \right)}}} \right\} + \left\{ {\frac{d}{{\left( {S - V} \right)}}} \right\} \cr & = 5\,{\text{hrs}}\,15\,{\text{minutes}} \cr & = \frac{{21}}{4}\,{\text{hours}}\,.......\left( 1 \right) \cr & {\text{and}}\left\{ {\frac{{2d}}{{\left( {S + V} \right)}}} \right\} = 7\,.....(2) \cr & {\text{Now}}, \cr & \frac{d}{{\left( {S + V} \right)}} = \frac{7}{4} \cr & Or,\,\frac{{2d}}{{\left( {S + V} \right)}} = \frac{7}{2} \cr}$$ Hence, he takes ⁷/₂ hours to row 2d km distance downstream.