A certain test preparation course is designed to help students improve their scores on the MCAT exam. A mock exam is given at the beginning and end of the course to determine the effectiveness of the course. The following measurements are the net change in 7 students' scores on the exam after completing the course 21,21,23, 20, 28, 22, 15 Using these data, construct a 90% confidence interval for the average net change in a student's score after completing the course. Assume the population approximately normal Step 1 of 4: Cakulate the sample mean for the given sample data. Round your answer to one decimal place Anowne Copy Data