Question 172
An $8 \times 8$ board is given.
a) What is the minimum number of squares we must mark on this board so that each of its $3 \times 3$ sub-boards has at least one marked square?
b) What is the minimum number of squares we must mark on this board so that each of its $3 \times 3$ sub-boards has at least three marked squares?
a) What is the minimum number of squares we must mark on this board so that each of its $3 \times 3$ sub-boards has at least one marked square?
b) What is the minimum number of squares we must mark on this board so that each of its $3 \times 3$ sub-boards has at least three marked squares?