Problem #PRU-66065

Problemas Métodos Ejemplos y contraejemplos. Pruebas constructivas Principio del casillero Principio del casillero (otro)

Problem

In each cell of a board of size \(5\times5\) a cross or a nought is placed, and no three crosses are positioned in a row, either horizontally, vertically or diagonally. What is the largest number of crosses on the board?