For questions $13$ to $17$: Consider the following system: Customers arrive to a bank with two tellers. Customers have an exponential inter$-$arrival time of $15$ minutes and exponential service time of $10$ minutes. What is the arrival rate for this system?

Group of answer choices

$4$ customers$/$hour

$10$ minutes

$15$ minutes

$6$ customers$/$hour

None of the other answers