A man crosses a river that flows at 3 m/s. He reaches a point directly opposite his starting point, covering a width of 60 m in 15 seconds. What is his swimming speed in still water?

12 m/s
13 m/s
5 m/s
10 m/s

Correct answer: 5 m/s

Solution

Width = 60 m, time = 15 s, so perpendicular velocity = 4 m/s. To reach directly opposite, the man's upstream velocity component must equal river speed = 3 m/s. His speed in still water = sqrt(4² + 3²) = 5 m/s.

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