I've had other interviewers ask me really abstract problems, such as how to calculate the number and types of elevators a particular-sized building needed.
The answer to that kind of question is the same kind of concept as the Drake equation: the point is not to give them a number, but to give them a formula for how one might arrive at the number. In your particular example, the answer might be: ([# of floors] * [# people per floor] * [# times each person uses the elevator]) / ([elevator capacity] * [elevator speed]), or something like that (since this is not an actual interview, I didn't bother to account for things like the fact that traveling to higher floors takes longer, etc.).
The interviewer is asking you the question so he can see how you approach the problem, not to see if you know trivia about elevator design. Do you go make a list of everything you think might matter first, or do you dive in and think up new factors on the fly? Did you leave something important out? Are you able to make even slightly reasonable estimates? (For example, "I figure there's about a million floors in a building and 1 person per floor, each of whom is constantly riding the elevator so you need 60,000 elevators per building" is probably not going to impress the interviewer.)