In past years, approximately 20% of all U.S. families purchased potato chips at least once a month.

Question:

In past years, approximately 20% of all U.S. families purchased potato chips at least once a month. You are interested in determining the fraction of all U.S. families that currently purchase potato chips at least once a month. How many families must you survey if you want to be 99% sure that your estimate of the relevant proportion is accurate within 2%?