5

Part of 1994 Spain Mathematical Olympiad

Problems(1)

21 pieces, bw, on the squares of a 3x7 rectangle

Source: Spanish Mathematical Olympiad 1994 P5

21

pieces, some white and some black, be placed on the squares of a

3\times 7

rectangle. Prove that there always exist four pieces of the same color standing at the vertices of a rectangle.

combinatoricsgridgeometryrectangle