Using Littles Law: L=W where: L is the average number of items (customers, dollars, etc.) in the system, is the arrival rate of items into the system, W is the average time a customer (item) spends in the system.

Suppose we are interested in estimating the average number of customers in the supermarket during the morning hours from 9 to 11am.

We collected the following information: on average there are 15 customers entering the supermarket every 5 minutes during that time.

on average a typical customer spent about 20 minutes in the supermarket about 20% of customers leave the supermarket without buying anything and the other 80% will go through the checkout area.

For those who need to pay, on average, a customer spends about 5 minutes in the checkout area