Question 6
An $n\times n$ sheet of stamps permits removal of blocks of three adjacent stamps in one row or one column. Tearing is allowed only along perforations separating adjacent stamps, and each block must come out in one piece. Let $b(n)$ be the smallest number of blocks that can be removed so that no further block can be removed. Prove that there are real constants $c,d$ such that
$$\frac17n^2-cn\le b(n)\le\frac15n^2+dn$$
for every positive integer $n$.
$$\frac17n^2-cn\le b(n)\le\frac15n^2+dn$$
for every positive integer $n$.