Q. A boat goes 20 km downstream in one hour and the same distance upstream in two hours. The speed of the boat in still water is:
Answer: 15 km/hr
Notes: Let the speed of boat in still water be x km/hr and the rate of stream be y km/hr. ∴ Downstream rate = (x + y) km/hr and upstream rate = (x - y) km/hr $latex Now, \frac{20}{x + y} = 1&s=1$ $latex => x + y = 20 .......\left ( 1 \right )$ $latex and \frac{20}{x-y} = 2&s=1$ $latex => x - y = 10 .......\left ( 2 \right )$ From (1) and (2) we have x = 15 km/hr. Hence option [D] is correct answer.

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