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    Question

    A boat takes a total of 6 hours to travel 45 km downstream

    and 30 km upstream. In another scenario, the same boat takes 8 hours to cover 60 km upstream and 30 km downstream. Determine the speed of the stream (in km/h).
    A 2.5 km/h Correct Answer Incorrect Answer
    B 3.5 km/h Correct Answer Incorrect Answer
    C 1.5 km/h Correct Answer Incorrect Answer
    D 4.5 km/h Correct Answer Incorrect Answer

    Solution

    Let the upstream and downstream speed of boat be 'U' km/h and 'D' km/h respectively.
    So, (45/D) + (30/U) = 6 --------- (I)
    And, (30/D) + (60/U) = 8 -------- (II)
    On solving 2 X equation I - equation II,
    We get, 2 X [(45/D) + (30/U) ] - [(30/D) + (60/U) ] = 2 X 6 - 8
    Or, (90/D) + (60/U) - (30/D) - (60/U) = 12 - 8
    Or, (60/D) = 4
    Or, 'D' = 15
    On putting value of 'D' in equation I,
    We get, (45/15) + (30/U) = 6
    Or, 3 + (30/U) = 6
    Or, (30/U) = 3
    Or, 'U' = 10
    Speed of stream = (1/2) X (downstream speed - upstream speed)
    = (1/2) x (15 - 10) = 2.5 km/h

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