SmithCo is a supplier to many brand name makers of mobile phones. SmithCo manufactures the external shells that enclose mobile phones. The strength of a shell is measured by applying increasing pressure to the shell and recording the pressure at which the shell breaks. A random sample of 50 shells made by SmithCo showed an average strength of 45.4 pounds and a sample standard deviation of 2.1 pounds.
(i) Construct a 95% confidence interval for the mean shell strength. What is the margin of error?
(ii) What should be the sample size for a margin of error of 0.2 pounds or less?