Maths Olympiad Prep

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Combinatorics Difficulty 5.6 AIME, harder Prove it

Along a round table are arranged 11 cards with the names (all distinct) of the 11 members of the 16th 16^{\text {th }} JBMO Problem Selection Committee. The distances between each two consecutive cards are equal. Assume that in the first meeting of the Committee none of its 11 members sits in front of the card with his name. Is it possible to rotate the table by some angle so that at the end at least two members of sit in front of the card with their names?

Solution

Yes it is: Rotating the table by the angles 36011,236011,336011,,1036011\frac{360^{\circ}}{11}, 2 \cdot \frac{360^{\circ}}{11}, 3 \cdot \frac{360^{\circ}}{11}, \ldots, 10 \cdot \frac{360^{\circ}}{11}, we obtain 10 new positions of the table. By the assumption, it is obvious that every one of the 11 members of the Committee will be seated in front of the card with his name in exactly one of these 10 positions. Then by the Pigeonhole Principle there should exist one among these 10 positions in which at least two of the 11(>10)11(>10) members of the Committee will be placed in their positions, as claimed.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.