If the cafeteria installs an automatic vendor that dispenses a cup of coffee at a constant time of 15 seconds, how many customers would you expect to see at the coffee urn (waiting and/or pouring coffee)

I suppose this question must have some distribution rate in order to find customers at coffee urn

A cafeteria has a coffee urn from which customers serve themselves; arrival at the urn follow a poisson distribution at the rate 3 per minutes. customer's takes about 15 second to serve themselves; exponential distribution.

a) The number of customers will be

Ls = λ / μ - λ

Where λ = Arrival rate=3/min, μ = Service rate=4/min

Ls = 3 customers would be expected at the on the average at the coffee urn (waiting or Pouring coffee)