A man can row at the speed of 8.5 km/h in still water. In a river running at the speed of 1.5 km/h, it takes him 51 minutes to row to a place and come back. How far (in km) is this place?

Correct option is D

Given:
Speed of man in still water = 8.5 km/h
Speed of river = 1.5 km/h
Total time taken= 51 minutes = $$\frac{51}{60} \text{ hours} = \frac{17}{20} \text{ hours}$$​​
Formula Used:
Speed downstream = Speed in still water + Speed of river
Speed upstream = Speed in still water - Speed of river
Solution:
Speed downstream = 8.5 + 1.5 = 10 km/h
Speed upstream = 8.5 - 1.5 = 7 km/h
Let the distance to the place be "D".
​$$\begin{align*}\text{Total time taken} &= \left( \frac{D}{\text{Speed downstream}} \right) + \left( \frac{D}{\text{Speed upstream}} \right) \\\Rightarrow \frac{17}{20} &= \frac{D}{10} + \frac{D}{7} \\\Rightarrow \frac{17}{20} &= \frac{7D + 10D}{10 \times 7} \\\Rightarrow \frac{17}{20} &= \frac{17D}{70} \\\Rightarrow D &= \frac{17}{20} \times \frac{70}{17} = 3.5\ \text{km}\end{align*}$$​​