A man rows to a place 48 km away and comes back to the starting point. The stream flows at 2 km/h and the whole trip takes him 10 hours. His speed in still water (in km/h) is:

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Accepted Answer

10. 48/(v + 2) + 48/(v − 2) = 10 → 96v = 10(v² − 4) → 5v² − 48v − 20 = 0 → v = (48 + 52)/10 = 10 km/h. (Check: 48/12 = 4 h down and 48/8 = 6 h back.) Forgetting the return leg gives 9.6, the average speed.

A man rows to a place 48 km away and comes back to the starting point. The stream flows at 2 km/h and the whole trip takes him 10 hours. His speed in still water (in km/h) is:

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