2010年Puzzleup数学竞赛−第02题: Round Table

castelu

Round Table

    A group of people is sitting around a round table. They give a coffee break. When they return to the table after the break, they sit down randomly. Interestingly, they notice that the six closest persons sitting next to each one of them (three to the left, and three to the right) are completely different from the six closest persons in the previous setting.

    At least how many persons should be there?