Question
A boat’s speed against the stream is 66.67% less than
its speed in still water. If it covers 180 km downstream in 4 hours, find the time (in minutes) required to go 90 km upstream.Solution
Let the speed of boat in still water be '6v' km/h Upstream speed = (1/3) × 6v = '2v' km/h Speed of stream = 6v − 2v = '4v' km/h Downstream speed = 6v + 4v = '10v' km/h So, 10v = 180 / 4 = 45 ⇒ v = 4.5 Upstream speed = 2 × 4.5 = 9 km/h Time taken = 90 / 9 = 10 hours = 10 × 60 = 600 minutes
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