Finitely many cells of an infinite square board are colored black. Prove that one can choose finitely many squares in the plane of the board so that the following conditions are satisfied: (i) The interiors of any two different squares are disjoint; (ii) Each black cell lies in one of these squares; (iii) In each of these squares, the black cells cover at least $\frac15$ and at most $\frac45$ of the area of that square.