How do I find the value that is one standard deviation below the mean?

Question

The following data are the distances between a sample of 20 retail stores and a large distribution center. The distances are in miles.

29; 30; 37; 40; 58; 67; 68; 69; 76; 81; 84; 95; 96; 96; 99; 106; 112; 127; 145; 150

Find the value that is one standard deviation below the mean. (Round your answer to one decimal place.)

How do I solve this? Do I do it on my calculator? please explain

Answer 1

First you find the mean and standard deviation

which per MS Excel are

1 standard deviation BELOW the mean is then 89.5625 - 1 * 34.45184 = 55.11066