A is walking toward a M$/$M$/1$ queueing system $($first$-$come, first$-$serve$).$ The customer arrival

rate at that system is $0\mathrm{.}4$ per minute, and the mean service time is $1$ minute per customer. B is

walking ahead of A on the way to the building and reaches it $2$ minutes before A does. At the

moment that A enters the building, A knows that nobody arrived there after B and before A$.$ A$,$

however, has no idea how many customers were present at the system when B arrived. Nor

does A know anything about the customer departure process: when services end, customers

leave the building from a different door that A cannot see. Given this information: