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BdMO National Secondary 2019#6

Posted: Thu Mar 14, 2019 10:59 am
by samiul_samin
Consider a $9\times 9$ array of white squares.Fond the largest $n\in \mathbb N$ with the property:no matter how one choice $n$ out of $81$ white squares and colour in black,there always remains a $1\times 4$ array of white squares(either vertical or horizontal).