假设某银行有四个窗口对外接待客户，从早晨银行开门起不断有客户进入银行。由于每个窗口在某个时刻只能接待一个客户，因此在客户人数众多时需在每个窗口前顺次排队，对于刚进入银行的客户，如果某个窗口的业务员正空闲，则可上前办理业务，反之，若四个窗口均有客户所占，他便会排在人数最少的队伍后面。现在需要编制程序以模拟银行的这种业务活动并计算一天中客户在银行逗留的平均时间。(Suppose a bank has four windows to receive customers, and from the morning bank opens, there are always customers entering the bank. Because each window to receive a customer at one time only, so in a large number of customers to line up in front of every window, for entering the bank customers, if a window salesman is free, you can come forward for business, on the other hand, if the four windows are for customers, he will be in the back row of the minimum number of teams. Procedures are now needed to model the business activities of the bank and calculate the average time of the customer's stay in the bank during the day.)