Question
A boat takes 7 hours to cover 273 km upstream and 9
hours to cover 441 km downstream. Find the time taken by the boat to cover 540 km upstream and 459 km downstream if speed of boat in still water is increased by 4 km/h and speed of stream is decreased by 2 km/h.Solution
Upstream speed = 273/7 = 39 km/h Downstream speed = 441/9 = 49 km/h Speed of boat in still water = (39 + 49)/2 = 44 km/h  Speed of stream = (49 – 39)/2 = 5 km/h Increased speed of boat in still water = 44 + 4 = 48 km/h Decreased speed of stream = 5 – 2 = 3 km/h Required time taken = 540/(48 – 3) + 459/(48 + 3) = 12 + 9 = 21 hours
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