Fifty students are enrolled in a business statistics class. after the first examination, a random sample of 5 papers was selected. the grades were 60, 75, 80, 70, and 90. a) determine the standard error of the mean. b) what assumption must be made before we can determine an interval for the mean grade of all the students in the class? explain why. c) assume the assumption of part b is met. provide a 90% confidence interval for the mean grade of all the students in the class.