Question
A boat takes 7 hours to cover 259 km upstream and 9
hours to cover 477 km downstream. Find the time taken by the boat to cover 645 km upstream and 660 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 = 259/7 = 37 km/h Downstream speed = 477/9 = 53 km/h Speed of boat in still water = (37 + 53)/2 = 45 km/h  Speed of stream = (53 – 37)/2 = 8 km/h Increased speed of boat in still water = 45 + 4 = 49 km/h Decreased speed of stream = 8 – 2 = 6 km/h Required time taken = 645/(49 – 6) + 660/(49 + 6) = 15 + 12 = 27 hours
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