Please answer this using formulas and not in excel Customers needing teller service arrive randomly at a mean rate of 30

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Please answer this using formulas and not in excel
Customers needing teller service arrive randomly at a mean rateof 30 perhour. Customers wait in a single line and are served by the nextavailable teller when they reachthe front of the line. Each service takes a variable amount of time(assume an exponentialdistribution), but on average can be completed in 3 minutes. Thetellers earn an average wage of$18 per hour.
(a)
Company policy is to have no more than a 10% chance that acustomer will need to wait morethan 5 minutes before reaching a teller. How many tellers need tobe used in order to meet thisstandard?