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    Question

    A man can go on a motorboat with a speed of 12 km/hour

    in still water to a certain upstream point and return to the starting point in a river that flows at a speed of 8km/hour. Find the average speed of the motorboat for the total journey. Type-ssc
    A 16/5 km/h Correct Answer Incorrect Answer
    B 17/3 km/h Correct Answer Incorrect Answer
    C 20/3 km/h Correct Answer Incorrect Answer
    D 10/3 km/h Correct Answer Incorrect Answer

    Solution

    Let the speed of the boat be b km/hour and the speed of the current be s km/hour. Average speed = 2xy/ (x+ y) (here, x is the speed of downstream and y is the speed of upstream)   = (2 (b +s) (b-s))/ ((b+ s +b -s)) = (b2 - s2)/b = (144 - 64)/12 = 80/12 = 20/3 Km/h

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