The expected waiting time in queue $($Wa$)$ for a G$/$G$/$m queuing model is estimated using Allen$-$Cunneen approximation.

a$)$ State the expression of the approximation.

b$)$ Apply your findings in a$)$ to the following situation:

A bank branch has $6$ tellers. At peak times, an average of $3,6$ customers per minute arrives at the bank. It takes a teller an average of $1$ minute to serve a customer. The squared coefficient of variation for both interarrival times and service times is $0\mathrm{.}5$

What is the average time a customer will have to wait before seeing a teller? On average, how many customers will be present in the system?