Q3. The amount of time that a drive-through bank teller spends on a customer is a random variable with a mean ? =3 .2 minutes and a standard deviation ? =1 .6 minutes. If a random sample of 64 customers is observed, ?nd the probability that their mean time at the tellers window 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.