I have to do some testing on a single server post office queuing system for my
assignment.  The testing could be like on average, how many people have to wait
for three mins. or how many people in the queue at any given tme etc.

I then need to perform graphs that shows the figure outputs (the graph itself
can be done in Excel)

Think of what states a customer going to the post-office can have.
He can:
1. add himself to a queue
2. wait in the queue
3. be served at the counter
4. leave the queue (after being served)

He can also:
5. switch to a shorter queue (with the same service)
6. leave the queue without being served
(it takes to long, come back later)

A. start with 1 queue
B. ignore 5. and 6.
C. define states:
customer_state=waiting,being_served, leaving
D. define the procedures:
add_customer, at_counter, remove_customer
(these procedures alter the state of a customer in the queue)
E. define variable: queue_length or something.
F. if it works, try for 2 or 3 queues

Does this makes sense / ring a bell?