While writing an article on the high cost of college education, a reporter took a random sample of the cost of new textbooks for a semester. The random variable x is the cost of one book. Her sample data can be summarized by n = 41 ∑x = 3582.17 and ∑(x - x)2 = 9960.336
a. Find the sample mean,  .
b. Find the sample standard deviation, s.
c. Find the 90% confidence interval to estimate the true mean textbook cost for the semester based on this sample.