Confidence interval for mean ..

A random sample of 81 credit sales in a department store showed an average sale of $68.00. Historically, it is known that the standard deviation of the population is $27.00. a) determine the standard error of the mean, b) with a 0.95 probability, what can be said about the margin of error and c) what is the 95% confidence interval of the population mean?

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...tandard deviation of the population is $27.00.
a) Determine the standard error of the mean,
Standard error of mean = =27/9 =3
b) With a ...