10. Draw a diagram! Two ferries leave simultaneously from opposite banks of a river and cross it perpendicular to the banks. The speeds of the ferries are constant. The ferries meet each other 720 meters from the nearest bank. Upon reaching the bank, they immediately head back. On the return trip, they meet 400 meters from the other bank. What is the width of the river?
10. The total distance they traveled by the time of their first meeting (720 m from one of the banks) is equal to the width of the river. When they meet for the second time, the total distance is three times the width of the river, which required three times the time. By the time of the first meeting, one of the ferries had traveled 720 m, and by the time of the second meeting - 2160 m (three times the distance). But this distance exceeds the width of the river by 400 m. Therefore, the width of the river is 1760 m.