The janitor of an apartment block, notices that the boiler pressure is rising, and will shortly explode.
He phones two of the residents living in 1 and 2, tells them the news, asks them to do the same and phone just two more people, (1 to call 3, 4 and 2 to call 5,6 and so on) and then get out of the building.
Assume that each of the residents is in, and answers the phone immediately. Each apartment has 1 resident.
If each phone call takes 30 seconds, and it takes each person 90 seconds to get out of the building, what is the minimum number of minutes it will take to empty all 375 apartments?