We can calculate the average unit rate in customers per hour dividing the number of customers the store has in a certain period of time by the number of hours of that period of time.
In this case we have 120 customers in a 6-hour period.
We can then calculate the rate as:
[tex]r=\frac{120\text{ customers}}{6\text{ hours}}=20\text{ customers/hour}[/tex]