Question 11
A $10 \times 10$ square is divided into $1 \times 1$ squares. A *point light* at a vertex of a $1 \times 1$ square lights all the $1 \times 1$ squares to which the vertex belongs. (A point light can be positioned on a vertex on the edge of the big square.) Find the minimum number of point lights required such that all the squares are lit even if one of the point lights is not functioning.
(Proposed by B. Battsengel)
(Proposed by B. Battsengel)