CombinatoricsDifficulty 4.8Prove itBelarus — Selection and Training Session · Belarus
A rectangle R with odd integer side lengths is divided into small rectangles with integer side lengths. Prove that there is at least one rectangle among the small rectangles whose distances from the four sides of R are either all odd or all even.
This one wants a proof. Work it on paper, then read the official solution and mark yourself. Be honest about it: the record is only any use to you if it is.