Please answer this using formulas and not on excel

Sally Gordon has just completed her MBA degree and is proud to have earned a promotion to Vice President for Customer Services at the First Bank of Seattle. One of her responsibilities is to manage how tellers provide services to customers, so she is taking a hard look at this area of the banks operations. 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.

Please answer this using formulas

(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?