Square tiles 9-inches on a side cover the floor of a rectangular room exactly. the border tiles are white and all other tiles are blue. the room measures 18 feet by 15 feet. how many tiles are white

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