The delivery times for all food orders at a fast-food restaurant during the lunch hour are normally

Question:

The delivery times for all food orders at a fast-food restaurant during the lunch hour are normally distributed with a mean of 7.7 minutes and a standard deviation of 2.1 minutes. Find the probability that the mean delivery time for a random sample of 16 such orders at this restaurant is
a. Between 7 and 8 minutes
b. Within 1 minute of the population mean
c. Less than the population mean by 1 minute or more