The bed of a river of width D is dry, so that you can walk across it. Two islands stick up out of the river bed -- one is directly downstream from the other, separated from the first by the same distance D.

Suppose that you walk at a speed of v. How long will it take to walk from A on the left bank to B on the right bank?

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Now, suppose you start on island X and walk to the other island, Y. How long will that take?

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Plug in some numbers: suppose that D = 2 km, v = 1 m/s. What are the times required for the two round trips?

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