Let the speed of boat in still water and speed of stream be 'x' km/hr and 'y' km/hr, respectively. According to question; (200/x) - (100/x) = 5 Or, (100/x) = 5 So, x = 20 Now, 9 x (20 + y) = 9 x (20 - y) + 18 Or, 180 + 9y = 180 - 9y + 18 Or, 18y = 18 So, y = 1 So, downstream speed of the boat = (20 + 1) = 21 km/hr And, upstream speed of the boat = (20 - 1) = 19 km/hr Required time taken = (250/21) + (250/19) ~ (12 + 10) ~ 22 hours
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