Question

    A boat can go 15 km upstream and 21 km downstream in 2

    hours. If the speed of the stream is 3 km/h, then the time taken (in hours) by the boat to go 21 km upstream and 15 km downstream is:
    A 2(4/35) Correct Answer Incorrect Answer
    B 3 Correct Answer Incorrect Answer
    C 3(/1/4) Correct Answer Incorrect Answer
    D 2(1/2) Correct Answer Incorrect Answer

    Solution

    Let the speed of boat in still water =B

    Let the speed of stream in still water =S

    Speed of boat in downstream =B+S

    Speed of boat in upstream =B-S

    B+S=21/2=10.5km/h

    B-S=15/2=7.5km/h

    Solving after –

    B=9km/h and S=1.5km/h

    ATQ-

    S =1.5 =3km/h

    B=9=18 km/h

    Now total taken time =

    (21/B-S) +(15/B+S)

    = (21/15) +(15/21)

    =74/35

    =2(4/35) hours

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