At a post office, customers arrive at an average rate of $18$ customers per hour following a Poisson distribution. The store currently has one clerk who can service an average rate of $20$ customers per hour $($assume Exponential distribution$).$

If the manger is considering adding an extra attendant to improve the system performance. If a clerk is paid $$30$ per hour while customer waiting is estimated at $$20,$ would it be more cost effective to have one or two clenks at the office?