3. (2 points) A chess piece called a quadraliner attacks two verticals and two horizontals adjacent to the square it occupies. What is the maximum number of non-attacking quadraliners that can be placed on a $10 \times 10$ board?
Answer: 25

## Solution:

Example: place the four-line rulers at the intersections of even columns and even rows.

Evaluation: In each pair of rows, the four-line rulers can only stand on one row. At the same time, they cannot stand on two adjacent cells.