Question
In still water, a boat can cover 30 km in 90 minutes.
Speed of the stream is 20% of the speed of the boat in downstream. Find the time taken by the boat to cover 18 km in upstream.Solution
ATQ, Let the speed of the stream be 's' km/h. Speed of the boat in still water = (30/90) X 60 = 20 km/h So, downstream speed of the boat = '(20 + s)' km/h ATQ, Speed of stream = 's' = 0.20 X (20 + s) Or, s = 4 + 0.2s Or, s - 0.2s = 4 Or, 0.8s = 4 So, 's' = 5 km/h Upstream speed of boat = 20 - s = 20 - 5 = 15 km/h Required time = 18 ÷ 15 = 1.2 hours
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