The SEC requires companies to file annual reports concerning their financial status. It is impossible to audit every account receivable. Suppose an auditor audits a random sample of 25 accounts receivable invoices and finds a sample average of $142 and a sample standard deviation of $30.

Part c) How large a sample (approximately) is required for you to be 95% sure that the estimate of the mean invoice size is accurate within about $10 ( the entire confidence interval length, from the lower limit to the upper limit)?