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    Question

    In still water, the speed ratio of boats A and B is

    10:7. Boat A’s still-water speed is twice the stream speed. If A covers 180 km downstream in 6 hours, how long will B take to go 72 km upstream?
    A 12 hrs Correct Answer Incorrect Answer
    B 18 hrs Correct Answer Incorrect Answer
    C 20 hrs Correct Answer Incorrect Answer
    D 10 hrs Correct Answer Incorrect Answer
    E None of these Correct Answer Incorrect Answer

    Solution

    ATQ, Let the speeds be 10x and 7x km/hr Stream speed = 10x/2 = 5x Downstream speed of A = 10x + 5x = 15x 180/6 = 30 ⇒ 15x = 30 ⇒ x = 2 Required time = 72/(7x – 5x) = 72/(2x) = 72/4 = 18 hours

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