MathDB
Colorful trapezoid

Source: 2024 CTST P19

March 28, 2024
combinatoricstrapezoid2024 CTST

Problem Statement

nn is a positive integer. An equilateral triangle of side length 3n3n is split into 9n29n^2 unit equilateral triangles, each colored one of red, yellow, blue, such that each color appears 3n23n^2 times. We call a trapezoid formed by three unit equilateral triangles as a "standard trapezoid". If a "standard trapezoid" contains all three colors, we call it a "colorful trapezoid". Find the maximum possible number of "colorful trapezoids".