The image of the public library is constantly changing, and their online services continue to grow. Usage of the library's home page grew by 17% during the past 12 months. It has been estimated that the current average length of a visit to the library's homepage is approximately 20 minutes. The library wants to take a sample to statistically estimate this mean. How large will the sample need to be to estimate the mean within 0.2 of 1 standard deviation with 0.95 confidence? (Round you answer up to the nearest whole number.)
Can someone help with formula for this and what do you use the percentage for on this one?