queueing problem probability

Problem #2 (20 points) - Using Queueing Theory For Planning Prior to introducing their new telephone exchange system, the local telephone company must plan their system capacity in terms of the number of calls that can be handled simultaneously. During peak times, calls are expected to come in at a rate of 6000 per hour, according to a Poisson process and calls are expected to last for an average of 20 minutes. If the telephone company wants to have adequate capacity more that 97.5% of the time, determine the minimum number of calls they must plan to handle each hour