A survey of 21 college freshmen found that they average 6.23 hours of sleep each night. A 99% confidence interval had a margin of error of 0.504. a. What are the lower and upper limits of the confidence interval? b. What was the standard deviation, assuming that the population standard deviation is known? . . . a. The lower limit of the confidence interval is and the upper limit of the confidence interval is (Round to three decimal places as needed.)