Problem

Source: INMO 2020 P6

Tags: Tiling, combinatorics, Coloring



A stromino is a $3 \times 1$ rectangle. Show that a $5 \times 5$ board divided into twenty-five $1 \times 1$ squares cannot be covered by $16$ strominos such that each stromino covers exactly three squares of the board, and every square is covered by one or two strominos. (A stromino can be placed either horizontally or vertically on the board.) Proposed by Navilarekallu Tejaswi