How large a sample size is needed to estimate the mean annual income of people in a certain county, correct to within $700 with probability 0.99?

How large a sample size is needed to estimate the mean annual income of people in a certain county, correct to within $700 with probability 0.99? No information is available about the standard deviation of their income. It is estimated that nearly all of the incomes fall between$0 and $180,000

and that this distribution is approximatelybell-shaped.