The shape of the distribution of the time required to get an oil change at a 15 minute facility is unknown. However, records indicate that the mean time is 16.1 minutes and the standard deviation is 4.6 minutes.

a. To compute the probabilities regarding the sample mean using the normal model, what size sample would be required?
b. What is the probability that a random sample of n = 40 oil changes results in a sample mean time of less than 15 minutes?