If you are using whole tiles, and they're square, and the floor measures 720 by 525 cm, the largest tile is the largest common factor of 720 and 525. 525=25*21=3*5*5*7; 720=5*12*12=2*2*2*2*3*3*5. The LCF=15, so the largest tile has a side of 15cm. When we divide 720 and 525 by 15 we get 35 and 48 by taking out 3*5 from each set of factors and multiplying the remaining factors together (5*7 and 2^4*3). Now we know how many tiles will fill the length and width of the room, so the total number of tiles is the product of these: 35*48=1,680 tiles.