Zoë lives in Apartment 23.

• On the first floor, the apartment that is the mirror image of Apartment 4, Carl's apartment, is Apartment 7.
• On the second floor, the apartment directly above Apartment 7 is Apartment 15.
• On the third floor, the apartment directly above Apartment 15 is Apartment 23, Zoë's apartment.
• (Add 8 to each apartment number on the first floor to get the second-floor apartment number directly above it, and then add another 8 to get the apartment number of the third-floor apartment.)