The mean airfare for flights departing from Buffalo Niagara International Airport during the first three months of 2017 was $320.51. Assume the standard deviation for this population of fares is known to be $80. Suppose a random sample of 60 flights departing from Buffalo Niagara International Airport during the first three months of 2018 is taken. 

a. If the mean and standard deviation of the population of airfares for flights departing from Buffalo Niagara International Airport didn’t changed between the first three months of 2017 and the first three months of 2018, what is the probability the sample mean will be within $20 of the population mean cost per flight? 

b. What is the probability the sample mean will be within $10 of the population mean cost per flight?