Let the abbreviation PSLT stand for the percent of the gross family income that goes into paying

Question:

Let the abbreviation PSLT stand for the percent of the gross family income that goes into paying state and local taxes. Suppose one wants to estimate the mean PSLT for the population of all families in New York City with gross incomes in the range $70,000 to $80,000. If σ = 2.5, how many such families should be surveyed if one wants to be 90% sure of being able to estimate the true mean PSLT within .5?