He starts on the 1st floor. He then goes up 18 floors:
1 + 18 = 19
He took the stairwell down 3 floors
19 - 3 = 16
He then takes the elevator down 8 floors
16 - 8 = 8
He then takes the stairwell up 5 floors for his afternoon meeting
8 + 5 = 13
C. thirteenth is your answer
hope this helps