Question 34
Let $m$, $n$ and $r$ be positive integers with $n \ge 2$ and $1 \le r \le n-1$. Consider a square table of size $(mn+r) \times (mn+r)$. The table is covered by squares of size $n \times n$ with sides parallel to the sides of the table. Each unit square is covered at least once and some unit squares are covered multiple times. Find the minimum possible number of unit squares covered at least two times.