French pre-TST-2004/2005 #6
Source: Me
December 20, 2004
analytic geometry
Problem Statement
On each unit square of a square, there is a bettle. Simultaneously, at the whistle, each bettle moves from its unit square to another one which has only a common vertex with the original one (thus in diagonal). Some bettles can go to the same unit square.
Determine the minimum number of empty unit squares after the moves.
Pierre.