A large food-products company receives about 100,000 phone calls a year from consumers on its toll-free number. A computer monitors and records how many rings it takes for an operator to answer, how much time each caller spends "on hold," and other data. However, the reliability of the monitoring system has been called into question by the operators and their labor union. As a check on the computer system, approximately how many calls should be manually monitored during the next year to estimate the true mean time that callers spend on hold to within 3 seconds with 95% confidence? Answer this question for the following values of the standard deviation of waiting times (in seconds): 10, 20, and 30.