$5\times 7$ covered by triomino
Source: All-Russian Olympiad, Grade 9, Second Day, Problem 8
April 18, 2013
combinatoricscheckerboard
Problem Statement
Can a checkerboard be covered by L's (figures formed from a square by removing one of its four corners), not crossing its borders, in several layers so that each square of the board is covered by the same number of L's?M. Evdokimov