In order to determine the average weight of carry-on luggage by passengers in airplanes, a sample of 25 pieces of carry-on luggage was collected and weighed. the average weight was 18 pounds. assume that we know the standard deviation of the population to be 7.5 pounds. determine the standard error of the mean.
a. 1
b. 1.5
c. 2
d. 2.5