These are pretty similar:
The population standard deviation of the number of people who shop online is 3.1. If you want to be 99% confident that the sample mean is within 2% of the population mean, how many people must be surveyed.
You're collecting information on salaries of IT workers. You know the population standard deviation of the salaries is $2125. You want to be 99% confident you are within $750 of the population mean. How many people do you need to survey?