He phones two of the residents, tells them the news, asks them to do the same and phone just two more people, and then get out of the building.
Assume that each of the residents is in, and answers the phone immediately.
If each phone call takes 30 seconds, and it takes each person 90 seconds to get out of the building, how long will it take to empty all 375 apartments?
Solution of Todays Puzzle 10-12-2011