answer is 3 how do you solve

The following build-up diagram tracks the number of customers for a single-server queueing system (e.g-t Question 19 an ATM machine) from 10:00am to 10:15am on a certain day. The number of customers shown in the chart includes both those in service and waiting in line. Build-up diagram f \# of Customers in system (in service + waiting) We will call the customer who arrived the earliest during this time span the first customer, the customer who arrived the second earliest the second customer, etc.. The service discipline is first-come-first-served (FCFS). According to the diagram, what is the service time of the first customer in minutes