I need some help can someone solve it or explain to me how to complete this exercise?

In a bank, there are 6 cashier desks where people come, are served, and leave. Customers enter 6 lines (queues) to get to the cashiers in an organised manner. The customer who enters each queue first, will be served first and leave the store. The person entering each of the queue next will be served after the person in front of him. In this way, the person entering each of the queue last will be served last.

Each customer also has the option to leave a cashier and thus leave the store, change queue or ask to be served first.

The number of customers that a cashier is able to serve per day/ week is 50.