Solution

ID: past-exam-of-the-mathematics-course-of-the-year-1-of-the-natural-sciences-course-of-the-university-of-cambridge/2010/paper-1/12/b/solution

After choices, let be the number of forward moves. The two unwrapped positions are and , whose gap is . It follows that the first subsequent meeting occurs exactly at step , regardless of the order of the choices. Its position is the number of forward moves in that block.
That count has a binomial distribution with parameters . Therefore
The two endpoint labels and represent the same physical seat reached by different complete circuits; the unwrapped convention distinguishes their probabilities.

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