interarrival times $($time between arrivals$)$ and the service times of $20$ customers have been

recorded and they are shown in the table below; the other columns in the table can be calculated

based on these two pieces of information. For example, the $$Arrival time$$ for the second

customer is the arrival time of the previous customer plus the time between the first and the

second customer $(18\mathrm{.}6+12\mathrm{.}7=31\mathrm{.}4).$ When this second customer arrives, the server is idle $($since

the first customer left at time $19\mathrm{.}0)$ and, consequently, his service begins immediately $($at his

arrival time $=31\mathrm{.}3)$; if the server had not been idle, he would have to wait for the previous

customer to leave the system. The second customer ends his service $($and leaves the system$)$ at

time $40\mathrm{.}9$ since he started his service at time $31\mathrm{.}3$ and his service took $9\mathrm{.}6$ time units. Thus, she

spent $40\mathrm{.}9-31\mathrm{.}3=9\mathrm{.}6$ time units in the system.