117. Two Ferries. Two ferries travel between two opposite banks of a river at constant speeds. Upon reaching a bank, each immediately starts moving in the opposite direction. The ferries departed from opposite banks simultaneously, met for the first time 700 meters from one of the banks, continued to their respective banks, then turned around and met again 400 meters from the other bank. Determine the width of the river in your mind.
117. Ferry $A$ departs from the shore, travels 700 m, and meets ferry $B$. By this time, they have covered a total distance equal to the width of the river. $A$ continues to the opposite shore, turns around, and after turning, travels another 400 m, meeting $B$ again. By this time, they have covered a total distance equal to three times the width of the river. Since their speeds are constant, $A$ has traveled a total distance of $3 \times 700 = 2100$ m. The width of the river is 400 m less than the distance traveled by $A$, which means it is 1700 m.

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[U. K. Rufus, A. M. M., 47, 111 (February 1940).]