Question
The speed of a boat going upstream is 20% slower than
when it is traveling downstream. The boat travels 90 km in still water over a period of 4 hours. Calculate the time required for the boat to cover distances of 120 km upstream and 125 km downstream.Solution
Speed of the boat in still water = 90/4 = 22.5 km/h Let the speed of the stream = x km/h So according to question: 0.80 × (22.5 + x) = 22.5 – x 18 + 0.8x = 22.5 – x 1.8x = 4.5, x = 2.5 Upstream speed of the boat = 22.5 – 2.5 = 20 km/h Downstream speed of the boat = 22.5 + 2.5 = 25 km/h Time taken by the boat to cover 120 km upstream and 125 km downstream = 120/20 + 125/25 = 6 + 5 = 11 hours
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