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?

____________________How long will it take to walk back, from B to A?

____________________How long will it take to make the round trip?

____________________

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

____________________How long will it take to walk back, from Y to X?

____________________How long will it take to make the round trip?

____________________

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

____________________ vs. _________________