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QUESTION $3.[20$ points$]$ The organizers of a conference in the Philadelphia Convention Center are evaluating the possibility of setting up a computer area where attendees can check their e$-$mail on computers provided by the organization. There will be one common queue for all computers, and only one person uses a computer at a time. On average, there are $15$ attendee arrivals per hour, the standard deviation of the time between arrivals is $4,$ and the average time a person spends on the computer is $10$ minutes $($with standard deviation $3).$ To ensure that waiting times are not too long, the organizers want to ensure that the utilization of the computers doesn't exceed $90\%.$ At least how many computers do they need to have?