First part of that question...I need to calculate Standard error of mean??

A SRS of 20 commuters to the Los Angeles metropolitan area are selected and each is asked how far they commute work each day. In the sample, the mean distance is 64 miles and the standard deviation is 12 miles. Assume that in the population of all commuters to the LA metropolitan area, daily commuting distances follow a normal distribution, with some mean Mu.





The standard error of mean is:





A) 14.31


B) 2.68


C) 7.20


D) 0.60

First part of that question...I need to calculate Standard error of mean??
The standard error of the mean is the standard deviation divided by the square root of the sample size:





se = sd/sqrt(n)


se = 12/sqrt(20)


se = 2.68 approximately





so B) is the correct answer.