New Enroll in RBI Grade B New Batch - Starts on May 21

    Question

    Ricky can go on a motor boat with a speed of 15 km/hour

    in still water to a certain upstream point and come back to the starting point in a river which flows at a speed of 5km/hour. Find the average speed of motor boat for total journey?
    A 11(1/3) km/hour Correct Answer Incorrect Answer
    B 13(1/3) km/hour Correct Answer Incorrect Answer
    C 11(2/3) km/hour Correct Answer Incorrect Answer
    D 13(2/3) km/hour Correct Answer Incorrect Answer
    E None of these Correct Answer Incorrect Answer

    Solution

    Let, the speed of boat be b km/hour and speed of current be s km/hour Average speed = 2xy/(x+y) (here, x is speed of downstream and y is speed of upstream) = (2 (b+s)(b-s))/((b+s+b-s)) = (b2-s2)/b = (225-25)/15 = `13 1/3` km/hour

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