Background data: The Arbograss report confirmed that water usage of...

Background data: The Arbograss report confirmed that water usage of guest showers was also normally
distributed with a mean of 30 gallons and a standard deviation of 10 gallons. 

Question: Suppose that Bates wants to collect his own data on water usage. He wants to
compute a 95% confidence interval of the form ± 1 gallon for the true mean water usage
of a shower. Assuming Arborgrass' estimate of standard deviation is accurate, how many
showers must Bates monitor in total to be able to compute such a confidence interval? 

Please use excel to generate the answer. Please show the excel formulas and calculations used to get the answer.