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Math check

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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 the following. (Give your answers correct to two decimal places.)
n = 21, Óx = 3635.6, and Ó(x - x)2 = 9750.4
(a) Find the sample mean, x.
$ 173.12.

(b) Find the sample standard deviation, s.
$ 22.08 .

(c) Find the 90% confidence interval to estimate the true mean textbook cost for the semester based on this sample.
Lower Limit $
Upper Limit $

I can not get the limits to work out I have tried 4 times and I only have one more time. I have plugged information into calculator that worked on other limits but not this one.

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