If you're talking floor tiles as opposed to wall tiles and by 12x12 you mean they are 12 inches square, then the answer is 300. Because each tile is one square foot and a 15'x20' room is 300 square feet. Simple, eh?
80 if the tiles are 12x12 - 12x12 is one square foot covering 80 you would need 80 tiles
375
30 but get 4 extra in case you break some
Each tile is one square foot. So you need 126 of them, but allow for spoilage.
36 tiles. I would hire a tiler if I was you, this doesn't sound like your 'thing'
forty
80 if the tiles are 12x12 - 12x12 is one square foot covering 80 you would need 80 tiles
256 tiles
The tile is bigger than the area that you want to cover (12x12 > 10x11) so one tile will do. <><><><><> However, if you meant 12x12 INCH tiles, and a 10x11 FOOT room, you would need 110 tiles.
13 boxes with 13 tiles in each
585
168
144
650
375
549!
If the tiles are 12x12 inches square - you would need... 75 !