The amount of time that a drive-through bank teller spends on a customer is a random variable with a mean µ = 3.2 minutes and a standard deviation s = 1.6 minutes. If a random sample of 64 customers is observed, find the probability that their mean time at the teller's counter is
a) At most 2.7 minutes:
b) More than 3.5 minutes;
c) At least 3.2 minutes but less than 3.4 minutes.