A certain test preparation course is designed to help students improve their scores on the GMAT exam. A mock exam is given at the beginning and end of the course to determine the effectiveness of the...

A certain test preparation course is designed to help students improve their scores on the GMAT 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 77 students' scores on the exam after completing the course:

20,18,2,4,15,15,1020,18,2,4,15,15,10

Using these data, construct a 80% confidence interval for the average net change in a student's score after completing the course. Assume the population is approximately normal