7. The little animals in the forest went on an outing, forming a line 40 meters long, advancing at a speed of 3 meters per second. The little rabbit has something to do and needs to rush from the end of the line to the front, and then immediately return to the end. The rabbit's speed is 5 meters per second. So, after $\qquad$ seconds, the rabbit can return to the end.
【Answer】 25
【Analysis】Key point: Travel problem. Students who have done the real questions in the White Book will find that a similar question has been tested before in the Zhonghuan Cup. This question is a combination of meeting and catching up in travel problems. Drawing a line segment diagram will help you understand quickly.
From the end of the line to the front: This is a catching-up problem, time $=$ distance difference $\div$ speed difference, $40 \div(5-3)=20$ seconds
From the front of the line to the end: This is a meeting problem, time $=$ distance sum $\div$ speed sum, $40 \div(5+3)=5$ seconds Total time: $20+5=25$ seconds