The amount of time that a drive-through bank teller spends on a customer is a random variable with a mean μ=3.5 minutes and a standard deviation σ=1.8 minutes. If a random sample of 81 customers is observed, find the probability that their mean time at the teller's window is (a) at most 3.2 minutes; (b) more than 3.7 minutes; (c) at least 3.5 minutes but less than 3.9 minutes.