Queueing - Probability of N customers in system

In this video, we compute the probability of having exactly N customers in the system. By doing this for different numbers of customers, we can figure out the probability of having 0, 1, 2, 3, etc. Taking those together, we can find the probability of having 2 or less, etc. We can then use that to find the probability of have 3 or more, etc.