The shape of the distribution of the time required to get an oil change at a15 -minute oil-change facility is unknown. However, records indicate that the mean time is 16.8 minutes, and the standard deviation is 4.8 minutes. (a) To compute 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=35 oil changes results in a sample mean time less than 15 minutes?