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A man rows to a place at a distance of 60 km and back in 18 hours. He finds that he can row 15 km with stream at the same time as 12 km against the stream. Find the speed of the boat in still water.


1. 6.25 km/hr
2. 6.75 km/hr
3. 4.21 km/hr
4. 5 km/hr
5. 5.75 km/hr

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Best answer
Correct Answer - Option 2 : 6.75 km/hr

Concept Used:

Speed = Distance/Time

Calculation:

Suppose he rows 15 km downstream and 12 km upstream in x hours.

Speed(upstream) = 12/x km/hr and Speed(downstream) = 15/x km/hr

It is given that he can row to a place 60 km distant and back in 18 hours.

⇒ 60/(12/x) + 60/(15/x) = 18

⇒ 5x + 4x = 18

⇒ x = 2

Speed(upstream) = (12/x) km/hr = 12/2 = 6 km/hr

Speed(downstream) = (15/x) km/hr = 15/2 = 7.5 km/hr

∴ Speed of boat = ½(6 + 7.5) = 6.75 km/hr 

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