A motor boat has speed 30 km/hr in still water. It goes 60 km down stream and comes back in \frac{9}{2} hours. What is the speed of the stream?

Explanation

Let the stream's speed be v. Total time = \frac{60}{30+v} + \frac{60}{30-v} = \frac{9}{2}. Testing the options, if v=10: \frac{60}{40} + \frac{60}{20} = 1.5 + 3 = 4.5 = \frac{9}{2}. Thus, the stream's speed is 10 km/hr.

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