Question: In the middle of the forest lies a branch with a length of 1 meter, directed East-West. On the branch are randomly positioned 25 ants, each facing East or West. Ants walk 1cm per second. When two ants meet each other, both ants turn around and start walking in the other direction. How long should you wait, to be certain that ant #13 is off the branch?
Answer: Consider how much time is needed for all ants to walk off the branch. All ants have the same speed, hence they are interchangeable. Thus, when two ants meet it does not matter whether the ants both turn around, or actually pass each other. When ants are able to pass each other, it becomes straightforward to see that each ant needs at most 100 seconds to walk off the branch.
Source: Winkler.