Please answer this using formulas and not in excel

Customers needing teller service arrive randomly at a mean rate of 30 per
hour. Customers wait in a single line and are served by the next available teller when they reach
the front of the line. Each service takes a variable amount of time (assume an exponential
distribution), but on average can be completed in 3 minutes. The tellers earn an average wage of
$18 per hour.

(a)

Company policy is to have no more than a 10% chance that a customer will need to wait more
than 5 minutes before reaching a teller. How many tellers need to be used in order to meet this
standard?