(3) Suppose a customer who arrives at a bank will be serviced immediately by either teller A or teller B with an equal probability. Let X and Y denote the service times (in minutes) of A and B respectively, and assume that X and Y are iid random exponential variables with mean 1. The service of the bank is considered to be \"satisfactory\" (event 3) if the service for a customer is completed in no more than one minute, or \"unsatisfactory\" (event U) otherwise. (a) Construct a tree diagram