You are considering buying a new car, but first you want to determine the gas mileage your current car is getting to see if a new car will save money on gas. You measure gas mileage over your next 15 trips and find that the mean is 24.2 mpg with a standard deviation of 1.3 mpg.

Find the 90% confidence interval for your gas mileage. Show/explain your work, and identify the following:

a) The estimated population mean and degrees of freedom

b) The assumptions you are making for this calculation and whether these appear to be valid

c) The critical value (specify z or t)

d) The margin of error

e) The 90% CI and the interpretation of these results, both written as complete sentences.