How large a sample must be taken to be 99% confident that the estimate of the average time of long-distance calls is within 3 minutes of the true average time of long-distance calls of all customers?...

How large a sample must be taken to be 99% confident that the estimate of the average time of long-distance calls is within 3 minutes of the true average time of long-distance calls of all customers? A previous study indicated that the standard deviation for long-distance calls is 20 minutes.