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 = 
<gadget id="calculator">9-7</gadget>
<output>2</output>
2nd floor.
Then went up to the 2 + 3 = 
<gadget id="calculator">2+3</gadget>
<output>5</output>
5th floor.
Then, went up to the 5 + 8 = 
<gadget id="calculator">5+8</gadget>
<output>13</output>
13th floor; hence there are 13 floors in the building.

<result>13</result>