When customers phone an airline to make reservations, they usually find it irritating if they are kept on hold for a long time. In an effort to determine how long phone customers are kept on hold, one airline took a random sample of 167 phone calls and determined the length of time each caller was kept on hold. The sample standard deviation was 3.8 minutes. How many more phone customers should be included in the sample to be 99% sure that the sample mean of hold times is within 30 seconds of the population mean.