An elevator is on the 9th floor. It goes down 7 floors, then up 3 floors, then up 8 floors. If the elevator is on the top floor, how many floors are there in the building? Let's be accurate as possible.
From the 9th floor, the elevator went down to the 9 - 7 = 2nd floor. Then went up to the 2 + 3 = 5th floor. Then, went up to the 5 + 8 = 13th floor; hence there are 13 floors in the building.
The answer: 13.