Students may choose between a 3-semcstcr-hour course in physics without, labs and a 4-semester-hour course with labs. The final written examination is the same for each section. If 12 students in the section with labs made an average examination grade of 84 with a standard deviation of 4, and 18 students in the section without labs made an average: grade of 77 with a standard deviation of 6, find a 99%. confidence interval for the difference between the average grades for the two courses. Assume the populations to be approximately normally distributed with equal variances.