please do only b part ?

The amount of time that a drive-through bank teller spends on a customer is a random variable with a mean u = 3.2 minutes and a standard deviation - 1.6 minutes. If a random sample of 64 customers is observed, find the probability that their mean time at the teller's 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