A bank installs an ATM and observes that customers arrive at a rate of 15 customers per hour on average and the arrivals follow a Poisson distribution (the interval time between two customers has an exponential distribution). The ATM has a fixed constant service time of 3 minutes per customer. 

 (a) Calculate the average time spent by a customer in this system. (b) What is the percentage of time that the ATM is idle? (c) The bank adds additional features to their ATM so customers can do different transactions at the ATM. With the upgrade the service time has now changed. The average is 3.75 min, with a std deviation of 2.12. What would the expected queue length be? And how long are customers waiting for? (d) With the data and answer from part c, if the bank decides to add another similar ATM machine, still maintaining a single queue, would they be able to get the wait times to less than a minute?