At Joe's Pool Hall They Rent Tables by the Hour

Multiple Choice

At Joe's Pool Hall they rent tables by the hour and also rent Balabushka pool cues for purists that can't afford their own. The rental process for these cues is exhaustive and takes an hour and a half for each application. There are three leasing agents that conduct these interviews, and on average, it is one hour between customer arrivals.
-What percentage of the time is the system completely empty?

A) 9%
B) 13%
C) 17%
D) 21%