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    A boat covers 780 km downstream in 15 hours. If the speed of the boat in still water is decreased by 20%, then the time taken by the boat to cover 624
    Question



    A boat covers 780 km downstream in 15 hours. If the speed of the boat in still water is decreased by 20%, then the time taken by the boat to cover 624 km upstream in 24 hours. Find the speed of the stream (In km/hr).

    A.

    24.33

    B.

    32.67

    C.

    43.33

    D.

    8.67

    E.

    None of these

    Correct option is D

    Information Given:
    A boat covers 780 km downstream in 15 hours.
    The speed of the boat in still water is decreased by 20%.
    The time taken by the boat to cover 624 km upstream in 24 hours.
    Formulas Used:
    Downstream speed = Speed of boat in still water + Speed of current
    Upstream speed = Speed of boat in still water - Speed of current
    Basic Explanation:
    Let the speed of boat in still water and speed of current be X km/hr and Y km/hr respectively.
    Downstream speed = 78015\frac{780}{15}​ = 52 km/h
    X + Y =52 …..(I)
    New speed of the boat in still water =X×80100X \times \frac{80}{100}​= 0.8X km/h
    ATQ,
    62424=0.8XY\frac{624}{24} = 0.8X - Y
    26 = 0.8X – Y…..(II)
    From I & II
    X = 1303\frac{130}{3}

    Y = 263\frac{26}{3}
    The speed of the stream = 263\frac{26}{3}​ = 8.67 km/h

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