# Problem №4

Two ferries simultaneously depart from opposite banks of a river and cross it perpendicularly to the banks. The speeds of the ferries are constant but not equal. The ferries meet at a distance of 720 meters from one bank, after which they continue their journey. On the return trip, they meet 400 meters from the other bank. What is the width of the river?
Answer: $1760 \mathrm{M}$

## Solution.

The total distance traveled by the ferries by the time of their first meeting is equal to the width of the river, and the distance traveled by the time of their second meeting is three times the width of the river. Therefore, by the time of the second meeting, each ferry has traveled three times the distance it traveled by the time of the first meeting. Since one of the ferries traveled 720 meters by the time of the first meeting, it traveled a distance of $720 \cdot 3=2160$ meters by the time of the second meeting. By this time, it had traveled a distance equal to the width of the river plus an additional 400 meters. Therefore, the width of the river is $2160-400=1760$ meters.

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