Problem: Consider a post office that is run by two clerks.
           Suppose that when Mr. Smith enters the system he  discovers that Mr. Jones is being served by one of the  clerks and
            Mr. Brown by the other.
            Suppose also that Mr. Smith is told that his service will  begin as soon as either Jones or Brown leaves.
            The amount of time that a clerk spends with a customer is   exponentially distributed with mean 1/λ

            What is the probability that, of the three customers, Mr. Smith is the last to leave the post office?