MathDB
24 convex quadrilaterals

Source: IMO Shortlist 2020 C2

July 20, 2021
IMO ShortlistcombinatoricsIMO Shortlist 2020quadrilateralcoloringspartitionGerhard Woeginger

Problem Statement

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.