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 whose corners are vertices of the 100-gon, so that[*] the quadrilaterals are pairwise disjoint, and
[*] every quadrilateral has three corners of one color and one corner of the other color.