A boat takes 7 hours to cover 294 km upstream and 9 hours to cover 486 km downstream. Find the time taken by the boat to cover 720 km upstream and 616 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.
Upstream speed = 294/7 = 42 km/h Downstream speed = 486/9 = 54 km/h Speed of boat in still water = (42 + 54)/2 = 48 km/h Speed of stream = (54 – 42)/2 = 6 km/h Increased speed of boat in still water = 48 + 4 = 52 km/h Decreased speed of stream = 6 – 2 = 4 km/h Required time taken = 720/(52 – 4) + 616/(52 + 4) = 15 + 11 = 26 hours
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