Question 62
Given a $3 \times 9$ array $A$ with each cell containing a positive integer, we say a $m \times n$ $(1 \le m \le 3,\ 1 \le n \le 9)$ subarray of $A$ is a “good rectangle” if the sum of the numbers in its cells is a multiple of $10$, and call a $1 \times 1$ cell of $A$ “bad” if it is not contained in any “good rectangle”. Find the maximum number of “bad cells” in $A$.