Correct Answer - Option 2 : 7 km/h
Given:'
The speed of the boat is x km/hr and the speed of the stream is 3 km/h.
He takes two and half times as long to row up as to row down the river.
Concept used :
Speed in downstream = Speed of boat in still water + Speed of stream
Speed in upstream = Speed of boat in still water + Speed of stream
Calculations :
Speed of boat in down stream = (x + 3) km/h
Speed of boat in upstream = (x - 3) km/h
According to the question
[(x + 3)/(x - 3)] = 5/2 (speed is inversly proportional to time)
⇒ 2x + 6 = 5x - 15
⇒ 3x = 21
⇒ x = 7 km/h
∴ Option 2 will be the correct choice.
