The results of a sample of 80 customers are:

Average amount spent:

Standard deviation:

Number who bought an item of supporter's apparel: 20.

The owner of a competing store decides to conduct his own survey.

c)What sample size does he need if he wants to estimate a 90% confidence interval for the mean amount spent to within , assuming a standard deviation of $8.50? How would the sample size change if he were to estimate it a 99% confidence interval instead?