Question
Ramesh rows in still water at the speed of 8 kmph to go a certain place and come back. Find his average speed for the whole journey if the river is flowing at a speed of 4kmph?
Solution
Ramesh’s speed upstream = 8 – 4 = 4kmph Ramesh’s speed downstream = 8 + 4 = 12kmph Let the distance be x km. ∴ Time taken upstream = x/4 kmph And time taken downstream = x/12 kmph ∴ Average speed = 2x/(x/4+ x/12) = 6 kmph Alternate method: Average speed during whole journey = (U^2-V^2)/U= (8^2-4^2)/8=(64-16)/8=48/8=6kmph
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