Q. 534.0( 20 Votes )

A motorboat whose speed is 9 km/hr in still water, goes 15 km downstream and comes back in a total time of 3 hours 45 minutes. Find the speed of the stream.

Answer :

Let the speed of stream be x km/h

Speed of boat is 9km/hr


Speed downstream = (9 + x)km/h


Speed upstream = (9 – x)km/h


Distance covered downstream = Distance covered upstream = 15km


Total time taken = 3 hours 45 minutes




taking LCM



81 – x2 = 72 cross multiplying


x2 = 81 – 72


x2 = 9 taking square root


x = 3 or – 3 (rejecting negative value)


Hence the speed of stream is 3 km/hr


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