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    Question

    A boat moves in a river where the ratio of the speed of

    the stream to the downstream speed of the boat is 1:6. If it takes the boat a total of 20 hours to travel 144 km downstream and return 144 km upstream, how much time will it take to cover 180 km in still water?
    A 22 hrs Correct Answer Incorrect Answer
    B 10 hrs Correct Answer Incorrect Answer
    C 12 hrs Correct Answer Incorrect Answer
    D 24 hrs Correct Answer Incorrect Answer
    E None of these Correct Answer Incorrect Answer

    Solution

    ATQ,

    Let the downstream speed of a boat and the speed of stream be 6x km/hr and 'x' km/hr respectively.

    Or, upstream speed of boat = 6x - x - x = 4x km/hr

    ATQ,

    (144/6x) + (144/4x) = 20

    Or, (24/x) + (36/x) = 20

    Or, (60/x) = 20

    So, 'x' = 3

    So, speed of boat in still water = 6x - x = 5x = 5 × 3 = 15 km/hr

    So, required time = (180/15) = 12 hours

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