Question: You want to obtain a sample to estimate a population mean. Based on previous evidence, you believe the population standard deviation is approximately ?=40.3. You would like to be 90% confident that your estimate is within 2.5 of the true population mean. How large of a sample size is required?  Edit

Answer: standard deviation ?= 40.3
margin of error E = 2.5
Z value for 90% CI=1.64
Sample size n= (Z*?/E)^2
=(1.64*40.3/2.5)^2
=698.9 ~699  Edit