# 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.

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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