I want to solve the following problem:
A dentist works 4 hours a day. Patients arrive on the average of 1 per 20 minutes and one patient spends on average 15 minutes with the dentist. Both time intervals have an exponential distribution. with
What is the probability that someone has to wait, and how much on average?
When can the dentist go home?
I figured out the parameters of the exponential distributions: $\lambda=3$ and $\mu=4$. But I don't know how to proceed. Thanks in advance!