There are 2.25 square feet in 1 18" x 18" tile.
Counting for NO waste, you would been 71 tile. Since tile this large with surely have some waste you will want to buy around 78 tile.
160 of them.
It is not clear what is meant by an "18x18 square feet" tile. 18ft * 18 ft = 324 square feet, so that one tile will be more than enough. An 18 inch by 18 inch tile cannot be a 18x18 square feet tile.
2.25 square feet
The answer will depend on the units used for measuring the tiles which you have failed to specify.
To cover 720 sq feet of floor with 18" x 18" tile, you would need 320 tiles.
144/2.25=64. 18x18 tile cover 2.25 sq ft area.
You would need 800 tile assuming you had no waste at all. Since this is unlikely you will need 825 tile.
160 of them.
It is not clear what is meant by an "18x18 square feet" tile. 18ft * 18 ft = 324 square feet, so that one tile will be more than enough. An 18 inch by 18 inch tile cannot be a 18x18 square feet tile.
2.25 square feet
The answer will depend on the units used for measuring the tiles which you have failed to specify.
The diagonal of an 18x18 tile is 18*sqrt(2) = 25.46 units.
To cover 720 sq feet of floor with 18" x 18" tile, you would need 320 tiles.
if you are quoting inches, then a 18x18 tile measures 2.25 square feet. On that basis you would need 106.66 or 107 to the nearest.
144/(1.5*1.5) = 64 tiles, but if you are using this to determine how many are needed to tile a room, you need to add at least 10% for edging around the room. So, you probably need at least 71 or more tiles.
About 128.
You just need to know your square footage, so length x width add 10% if laying straight 15% if in a pattern. If you want to know exactly how many pieces you need then each 18x18 contains 2.25sf. So just divide your square footage by 2.25 Tip: If your length and width are in inches then after you add your overage divide by 144.