olymmath

ISL 2020 C2

    In a regular 100-gon, 41 vertices are colored black and the remaining 59 vertices are colored white. Prove that there exist 24 convex quadrilaterals Q1,…,Q24Q_{1}, \ldots, Q_{24} whose corners are vertices of the 100 -gon, so that - the quadrilaterals Q1,…,Q24Q_{1}, \ldots, Q_{24} are pairwise disjoint, and - every quadrilateral QiQ_{i} has three corners of one color and one corner of the other color. (Austria)

    Official text Transcription from NuminaMath olympiads, checked word for word against the official paper. Quoted with source.

    Official text: IMO 2020 Shortlist booklet, page 8.

    Solutions

    Official solution Solution 1 by the IMO Problem Selection Committee

    The terms of the source let olymmath link to this solution but not copy it. Read the solution (IMO official site).

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