A group of students wants to estimate the mean length of time (in minutes) the average U.S. adult spends on their cell phone each day. To determine this estimate, the students take a random sample of 20 U.S. adults and obtains the times (in minutes) below.

24 27 26 29 33 21 18 24 23 34 17 15 19 23 25 29 36 19 18 22

99% confidence interval for the population mean:

Calculate the margin of error