At a post office, customers enter a single line waiting to be served by any one of two clerks. Every minute there is a 60% chance that a new customer arrives. If there is no one in line and a server is free, the customer does not wait to be served. When a customer is being served there is a 25% chance every minute that they complete their business and leave. When the clerk is free he will take the next customer in line, in the order that they arrived. Every minute, there is a 5% chance that a person standing in line will give up and leave. The post office is always open (24/7/365).
What is the average amount of time a customer spends in the post office (including those not served)?