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    Question

    Motor boat covers a certain distance downstream  in 10

    hours. It covers the same distance upstream in 15 hours. If the speed of the water is 9 km/hr, then the speed of the boat in still water is:
    A 55 km/hr Correct Answer Incorrect Answer
    B 45 km/hr Correct Answer Incorrect Answer
    C 50 km/hr Correct Answer Incorrect Answer
    D 75 km/hr Correct Answer Incorrect Answer
    E 90 km/hr Correct Answer Incorrect Answer

    Solution

    Let the distance be ‘D’ km According to the question, Downstream speed  x + y = d/10 or d = 10 (x+y) .......... (i) Upstream speed x-y = d/15 or d = 15(x-y) ........... (ii) Comparing (i) & (ii) 10(x+y) = 15(x-y) 2(x+y) = 3(x-y) 2x + 2y = 3x - 3y x = 5y Hence, y = Speed of the current = 9 km/hour. Speed of boat, x = 5 × 9 = 45 km/hr

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