First we need to convert the square feet to square inches
1 square feet = 144 square inches
Therefore, 81 square feet = 81*144 = 11,664 square inch
Each square tile is 18 inches long. Since its a square, the area of each tile = 18*18 = 324 square inch
So,the number of tiles required to cover the entire floor= total area of the floor in square inches/ area of the tile in square inches
The number of tiles required to cover the entire floor = 11,664/324 = 36 tiles
The number of tiles required to cover the entire floor = 36 tiles