Consider a $5\times 5$ grid with $25$ cells. What is the least number of cells that should be colored, such that every $2\times 3$ or $3\times 2$ rectangle in the grid has at least two colored cells?
Source: KAMO 2022 Grades 7-8 P2
Tags: rectangle, combinatorics
Consider a $5\times 5$ grid with $25$ cells. What is the least number of cells that should be colored, such that every $2\times 3$ or $3\times 2$ rectangle in the grid has at least two colored cells?
