In order to estimate the average time spent per student on the computer terminals at a local university, data were collected for a sample of 81 business students over a one-week period. assume the population standard deviation is 1.8 hours. a) find standard of error of the mean? b) with a 0.95 probablity, what is the margin of error (approximately)? c) if the sample mean is 9 hours, then the 95% confidence interval is approx?