Combinatorics · Extremal combinatorics · Grids · Pigeonhole · Covering
Stars are drawn in the cells of a \(4 \times 4\) table, at most one star per cell. What is the least number of stars for which the following holds: whichever \(2\) rows and whichever \(2\) columns are erased, at least one star is still left in the table?
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Slovenian High School Mathematics Competition for Vega Awards (MaSSA), drzavno (national) round 2002, 1. letnik, category A, problem 4. Organized by DMFA Slovenije (Society of Mathematicians, Physicists and Astronomers of Slovenia). Source