The principal randomly selected six students to take an aptitude test. their scores were: 87.4 86.9 89.9 78.3 75.1 70.6 determine a 90% confidence interval for the mean score for all students. assume the population is normally distributed. give the lower limit of the interval as your answer below. round to two decimal places. (hint from class: note that you will need to calculate the sample mean (xbar) and sample standard deviation (s) for this sample data in order to find the confidence interval endpoints. in class i referred to the formula for that -- and erroneously gave the formula for the sample variance instead of the sample standard deviation. the sample standard deviation would be the square root of the sample variance. formulas for both can be found on the test formula sheets posted on ilearn among other places.)