first convert feet to inches:
18 ft × (12in/1ft)= 216 in
15 ft × (12 in/1ft)= 180 in
divide each side of the room by 9 inches to get number of white tiles per a wall
wall 1(18ft): 216in/9in= 24 white tiles
wall 2(15ft): 180in/9in= 20 white tiles
wall 3(18ft): 216in/9in= 24 white tiles
wall 4(15ft): 180in/9in= 20 white tiles
Lastly, add up all the white tiles and subtract 4 (the tiles that we counted twice because they were the corner tiles and touched both walls)
24+20+24+20-4= 84 white tiles total
