self Maths Puzzle

This guy living on the 20th floor in an apartment building got up early each morning to go to work in a downtown store. He always went into the elevator on the 20th floor and rode down to the entrance (1st floor). When he came home he always rode the elevator from the entrance and up to the 8th floor. He walked out of the elevator and walked the stairs up to his apartment on the 20th floor.

Why didn't he take the elevator all the way up to his apartment?

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self Other Question

A man while looking at a photograph said, "Brothers and sisters have I none. That man's father is my father's son." The population of town was 24000. If males increase by 6% and females by 9%, it becomes 25620. Number of males and females are
1.18000, 6000
2.19000, 5000
3.16000, 8000
4.20000, 4000