Altug's Coffee Shop decides to install an automatic coffee vending machine outside one of its stores to reduce the number of people standing in line inside. Mehmet Altug charges $ $3\mathrm{.}50$ ?per cup. However, it takes too long for people to make change. The service time is a constant $3$ ?minutes, and the arrival rate is $15$ ?per hour $($ Poisson distributed $).$ ?a $)$ ?The average wait time in the line $=\_\_\_\_$ minutes $($ round your response to two decimal places $).$