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    Question

    The speeds of boats A and B in still water are in the

    ratio 5:3. The upstream speed of A is the same as the downstream speed of B. If the stream flows at 4 m/s, find the average of their downstream speeds.
    A 42 m/s Correct Answer Incorrect Answer
    B 32 m/s Correct Answer Incorrect Answer
    C 20 m/s Correct Answer Incorrect Answer
    D 18 m/s Correct Answer Incorrect Answer
    E None of these Correct Answer Incorrect Answer

    Solution

    Let the speed of boat A in still water be 5x m/s. So, the speed of boat B in still water = 3x m/s. ATQ, 5x − 4 = 3x + 4 ⇒ 2x = 8 ⇒ x = 4 Downstream speed of boat A = 5x + 4 = 5 × 4 + 4 = 24 m/s Downstream speed of boat B = 3x + 4 = 3 × 4 + 4 = 16 m/s Required average = (24 + 16) / 2 = 20 m/s

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