The U.S. Energy Information Administration (US EIA) reported that the average price for a gallon of regular gasoline is $2.94. The US EIA updates its estimates of average gas prices on a weekly basis. Assume the standard deviation is $0.21 for the price of a gallon of regular gasoline and recommend the appropriate sample size for the US EIA to use if they want a margin of error with 95% confidence.

How big should the sample size be for a margin of error of $0.10.