MathDB

You have

2

columns of

11

squares in the middle, in the right and in the left you have columns of

9

squares (centered on the ones of

11

squares), then columns of

7,5,3,1

squares. (This is the way it was explained in the original thread, http://www.artofproblemsolving.com/Forum/viewtopic.php?t=44430 ; anyway, i think you can understand how it looks)
Several rooks stand on the table and beat all the squares ( a rook beats the square it stands in, too). Prove that one can remove several rooks such that not more than

11

rooks are left and still beat all the table.
Proposed by D. Rostovsky, based on folklore

Problem Statement