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    Question

    A boat can cover 90 km upstream and 225 km downstream in

    15 hours. Speed of the stream is how much less than the speed of the boat in still water if the boat can cover 72 km upstream and 180 km downstream in 12 hours?
    A 20 km/h Correct Answer Incorrect Answer
    B 16 km/h Correct Answer Incorrect Answer
    C 18 km/h Correct Answer Incorrect Answer
    D 15 km/h Correct Answer Incorrect Answer
    E None of these Correct Answer Incorrect Answer

    Solution

    ATQ, Let the upstream speed and downstream speed of the boat be x km/h and y km/h, respectively. So according to question: 90/x + 225/y = 15 ………………. (i) Also, 72/x + 180/y = 12 ……….. (ii) Solving (i) and (ii), we get x = 18 and y = 36 So, the upstream speed and downstream speed of the boat are 18 km/h and 36 km/h, respectively. Speed of the boat in still water = (18 + 36)/2 = 27 km/h Speed of the stream = (36 – 18)/2 = 9 km/h So, the desired difference = 27 – 9 = 18 km/h

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