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    Question

    Speed of a boat in still water to speed of boat in

    upstream is 7:4. If the boat can travel 450 km in downstream in 5 hours, then find the time taken by the boat to cover 21 km in still water.
    A 30 minutes Correct Answer Incorrect Answer
    B 20 minutes Correct Answer Incorrect Answer
    C 15 minutes Correct Answer Incorrect Answer
    D 18 minutes Correct Answer Incorrect Answer
    E None of these Correct Answer Incorrect Answer

    Solution

    Let speed of boat in still water and speed of boat in upstream be ‘7x’ km/h and ‘4x’ km/h, respectively. Speed of stream = 7x – 4x = 3x km/h Speed of boat in downstream = 7x + 3x = 10x km/h So, 10x = 450/5 = 90 Or, x = 9 Speed of boat in still water = 7 × 9 = 63 km/h Desired time = 21/63 = 20 minutes

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