A square tile has sides of 10 centimetres. A smaller square measuring 2 centimetres by 2 centimetres is cut away from one of its corners. What area of tile is left?

Explanation

The whole square covers 10 times 10, and the piece taken away covers 2 times 2, so 100 minus 4 is what remains. Taking away 2 instead of 4 confuses the length of the cut edge with the surface that the cut removes, and a length can never be subtracted from an area.