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?
From the 9th floor, the elevator went down to the 9 - 7 = <<9-7=2>>2nd floor.
Then went up to the 2 + 3 = <<2+3=5>>5th floor.
Then, went up to the 5 + 8 = <<5+8=13>>13th floor; hence there are 13 floors in the building.
#### 13