Question: A bank officer wants to determine the amount of the average total monthly deposits per customer at the bank. He believes an estimate of this average amount using a confidence interval is sufficient. How large a sample should he take to be within $200 the actual average with 99% confidence? He assumes the standard deviation of total monthly deposits for all customers is about $1,000.  Edit

Answer: Margin of error={sigma/sqrt(n)}*z_c [sigma=sd=1000, n=sample size, z_c=2.58 at 99% confidence, margin of error=200]

or, sqrt(n)=(1000/200)*2.58=12.9
n=166.41 =166

Sample size=166(Ans.)  Edit