Let the speed of boat in still water and speed of stream be 'x' km/hr and 'y' km/hr, respectively. According to question; (120/x) - (90/x) = 4 Or, (30/x) = 4 So, x = 7.5 Now, 4 x (7.5 + y) = 4 x (7.5 - y) + 8 Or, 30 + 4y = 30 - 4y + 8 Or, 8y = 8 So, y = 1 So, downstream speed of the boat = (7.5 + 1) = 8.5 km/hr And, upstream speed of the boat = (7.5 - 1) = 6.5 km/hr Required time taken = (200/8.5) + (120/6.5) ~ (24 + 19) ~ 43 hours
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