A room is shaped like the letter L. It would be a rectangle 9 metres by 6 metres if a rectangular corner piece measuring 3 metres by 2 metres had not been left out of it. What is the floor area of the room?

Explication

A full 9 by 6 rectangle would cover 54 square metres, and the missing corner accounts for 3 times 2 of them, so the floor comes to 54 minus 6. Taking away 3 plus 2 removes a pair of lengths rather than the surface those lengths enclose, and lengths and areas cannot be subtracted from one another.