The vending machine fills cups with 6 ounces of coffee. Fifty cups of coffee are checked, and we find an average of 5. 85 ounces and a standard deviation of 0. 18 ounces. How large of a sample do we need to be 95% confident (use za/2 of 1. 96) that the sample mean is within 0. 02 ounces of the population mean?