Problem 3. On a river, two identical sightseeing boats departed from one pier in opposite directions at 13:00. At the same time, a raft also set off from the pier. After an hour, one of the boats turned around and headed in the opposite direction. At 15:00, the second boat did the same. What is the speed of the current if, at the moment the boats met, the raft had drifted 7.5 km from the pier?
Answer: 2.5 km/h.

Solution. Consider the reference frame associated with the river. In this frame, the boats move with equal speeds: initially, they move away from each other in opposite directions for 1 hour, then move in the same direction for another 1 hour (during which the distance between them does not change), and then approach each other until they meet. Since the distance between the boats did not change during the second time interval, they will approach each other for as long as they moved away, which is 1 hour. Thus, from the moment of departure until the meeting, 3 hours have passed. During this time, the log (moving with the current) traveled 7.5 km. Therefore, its speed is 2.5 km/h.