Please help!! I have no idea how to do this question.

14. A sample of 40 CD’s from your collection showed a mean length of 52.74 minutes with a standard deviation of 13.21 minutes
a. Construct the 95 percent confidence interval for the mean.
b. Construct the 95 percent confidence interval for the mean using the t-value rather than the z-value. Comment on why the interval is larger when you use the t-value.
c. Why might normality of the data set be in question here? Comment on your answer.