1. The problem statement, all variables and given/known data Suppose the porosity (in %) of samples taken from the ground found to be normally distributed with σ = 0.85 % What sample size is necessary to estimate the true mean porosity to within 0.25 with 99% confidence? 2. Relevant equations C.I. = confidence interval = mean +- z*σ*n^(-0.5) n = (2*z*σ/w)^2 interval width = w z = 2.575 3. The attempt at a solution Not really sure what is meant by "... to within 0.25" maybe someone can help clarify this? Is it referring to the confidence interval width? Also, I thought that the confidence intervals do not estimate the true mean. I thought C.I. only estimates whether or not the other samples will have the same mean within the C.I. range.