Question 41790
You are on the right tract, but I think you just mixed up 540cm with 504 cm when you did your computations. Just start over again and enter 540cm X 405cm = 218700.
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Also, try dividing by 15^2 since the tile is a square. Therefore all of its sides are 15cm. 
Area = (length)(width) = 15 x 15 = 15^2 = 225:
218700/(15^2) = 972. 
Make the correction of n X 15 to n X 15^2 = 218700 and you should get the teacher's answer.