Combinatorics · Extremal combinatorics · Triangular grid · Packing and covering · Matchstick configurations
Eighteen matches are laid out to form the grid shown below: an equilateral triangle whose side is three matches long, divided into nine small triangles. What is the smallest number of matches that must be removed so that the matches left behind form no triangle at all?
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Slovenian High School Mathematics Competition for Vega Awards (MaSSA), drzavno (national) round 2017, 1. letnik, category A, problem B3. Organized by DMFA Slovenije (Society of Mathematicians, Physicists and Astronomers of Slovenia). Source