No opium-smoking in the elevators.
—sign in a New York City hotel (1907)
This is excellent advice, in general, and particularly so while studying the following problem, as you’ll need all your wits unfogged if you are to have any chance of success.
Elevators are a wonderful source of fascinating probabilistic questions. They are easy to understand—elevators, after all, even though they are built to go straight up (with the exception of the fascinating elevators in the Luxor casino and hotel in Las Vegas)—aren’t ...