A motorboat whose speed is 15 km/hr in still water goes 30km downstream and comes back in four and a half hours. The speed of the stream is ?

A. 4.5 km/hr

B. 6 km/hr

C. 7 km/hr

D. 5 km/hr

Answer : D. 5 km/hr

Explanation :

Let the speed of the stream be x km/hr. Then,
Speed downstream = (15 + x) km/hr
Speed upstream = (15 – x) km/hr

We know from question that it took 4(1/2)hrs to travel back to same point.
So, (30/15 + x) + (30/15 − x) = 4(1/2)
=> 900/225 − x2 = 9/2
=> 9x2 = 225
=> x = 5km/hr.

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