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    Question

    A Boat travel 80 km distance in downstream and 48 km

    upstream and the total time taken is 20 hours. The same boat travels 30 km distance in downstream as much time travel 12 km in upstream. Find the speed of boat in still water?
    A 10km/hr Correct Answer Incorrect Answer
    B 7km/hr Correct Answer Incorrect Answer
    C 9km/hr Correct Answer Incorrect Answer
    D 6km/hr Correct Answer Incorrect Answer
    E None of these Correct Answer Incorrect Answer

    Solution

    Let the speed of stream be x And speed of Boat be y Total time taken =80/(x+y) + 48/(x - y) = 20 ………………………. (i) And time taken for 30 km in downstream and 12 km in upstream is equal 30/(x + y) = 12/(x - y) 5x – 5y = 2x + 2y 3x = 7y  x/y = 7/3 Now x = 7k and y = 3k Put the value of x and y in equation (i), 80/(7k + 3k) + 48/(7k - 3k) = 20 80/10k  + 48/4k  = 20 8/k + 12/k = 20 20/k = 20 k = 1 Therefore the speed of Boat in still water is 7 × 1 = 7 km/hr

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