A cell phone service provider has selected a random sample of 20 of its customers in an effort to estimate the mean number of minutes used per day. The results of the sample included a sample mean of 34.5 minutes and a sample standard deviation equal to 11.5 minutes. The population is assumed to be normally distributed. Based on this information, the cell phone service provider wants to construct an interval for the true mean with 99% confidence level.
a) State the appropriate critical value to be used
b) Find the margin of error that will be reported
c) Find the lower limit of the confidence interval estimate
d) Find the upper limit of the confidence interval estimate
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