Q. A person can row a distance of one km upstream in ten minutes and downstream in four minutes. What is the speed of the stream?
Answer: 4.5 km/hr
Notes: Speed in still water = x km/hr Speed of current = y km/hr $latex \therefore x + y = \frac{1}{\frac{4}{60}} = 15&s=1$ $latex x-y = \frac{1}{\frac{10}{60}} = 6&s=1$ ∴ Speed of current = $latex = \frac{1}{2}\left [ \left ( x+y \right )-\left ( x-y \right ) \right ]&s=1$ $latex = \frac{1}{2}(15-6) = \frac{9}{2} = 4.5 km/hr&s=1$ Hence option [B] is correct answer.

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