A market researcher plans to sample sales receipts at a natural food store to estimate the average size (in dollars) of a customer purchase. Previous analysis suggests that the standard deviation of the purchase amount is approximately $25.
In order to calculate a 95% confidence interval of total width less than $5, how many sales records should the researcher include in her sample?
a) 97
b) 271
c) 385
d) The answer cannot be determined from the information provided.