The speed of a boat in still water is 15 km/hr. If it can travel 42 km downstream and 28 km upstream in the same time, then what is the speed of the stream?

Explanation

Let the speed of the stream be v. Time taken is equal, so \frac{42}{15+v} = \frac{28}{15-v}. Simplifying gives 3(15-v) = 2(15+v) \implies 45 - 3v = 30 + 2v \implies 5v = 15 \implies v = 3 km/hr.

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