The average natural gas bill for a random sample of 21 homes in a certain city during the month of march was $311.90 with a sample standard deviation of $51.60. what is the standard error of the mean for this sample?

Solution:  
Standard Error Formula:  
 SExĚ„ = S / UNDERROOT N  
 where, 
 SExĚ„ = Standard Error of the Mean  
 s = Standard Deviation of the Mean  
 n = Number of Observations of the Sample  
 SExĚ„ = 51.60/ âš21 = 11.26004314 
 So, standard error = 11.26004314

A. is the answer in this question