a previous analysis of paper boxes showed that the standard deviation of their lengths is 13 millimeters. a packer wishes to find the 95% confidence interval for the average length of a box. how many...

a previous analysis of paper boxes showed that the standard deviation of their lengths is 13 millimeters. a packer wishes to find the 95% confidence interval for the average length of a box. how many boxes does he need to measure to be accurate within 4 millimeters