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? 

Given:

Speed of man in still water = 8.5 km/h

Speed of river = 1.5 km/h

Total time taken = 51 minutes = 51/60 hours = 17/20 hours

Formula used:

Time = Distance/Speed downstream + Distance/Speed upstream

Speed downstream = Speed in still water + Speed of river

Speed upstream = Speed in still water - Speed of river

Calculation:

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".

Total time taken = (D / Speed downstream) + (D / Speed upstream)

⇒ 17/20 = D/10 + D/7

⇒ 17/20 = (7D + 10D) / (10 × 7)

⇒ 17/20 = 17D / 70

⇒ D = 3.5 km

∴ The correct answer is option 2.