# Inspired by Chung Kevin

Probability Level 3

There are nine $3 \times 3$ sub-grids in the $5 \times 5$ grid. If at most 5 out of 9 unit squares are shaded blue in any $3 \times 3$ sub-grid, what is the maximum number of unit squares in the $5 \times 5$ grid that can be shaded blue?

Note: An example of how the grid can be shaded is shown to the right.

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