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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