GMAT PREP PS - MEAN and SD

[rveeraga]

If a certain sample of data has a mean of 20 and standard deviation of 3, which of the following values is more than 2.5 standard deviations from the mean?

a. 12
b. 13.5
c. 17
d. 23.5
e. 26.5

answer is a. 12

Can some one shed light on, how to get the answer?

Thanks.

[satish.nagdev]

clear explanation given here https://www.manhattangmat.com/forums/if- ... t4559.html

Thanks Ron Purewal