Question 81
A $7 \times 7$ board is divided into $49$ squares. Onto this board we place several tiles like the one in the picture
(the tiles can be rotated),
each tile covering two squares. At least how many tiles do we need to place onto the board, so that every uncovered square will be adjacent to at least one covered square? (Two squares are adjacent if they share a common side.)
(the tiles can be rotated),
each tile covering two squares. At least how many tiles do we need to place onto the board, so that every uncovered square will be adjacent to at least one covered square? (Two squares are adjacent if they share a common side.)