A bank has two tellers working on savings accounts. The first teller handles withdrawals only while the second teller handles depositors only. It has been found that the service time distributions of both depositors and withdrawals are exponential with a mean service time of 3 minutes per customer.

Depositors and withdrawers are found to arrive in a Poisson process throughout the day with mean arrival rate of 16 and 8 per hour respectively. What would be the effect on the average waiting time for depositors and withdrawers if each teller could handle both withdrawals and deposits? What would be the effect if this could only be accomplished by increasing the service time to 4 minutes?