Students who took an introductory statistics course can be divided into two groups; those who attended class regularly and those who did not.

Independent random samples of 20 students were taken from each group, and their midterm exam score was recorded. The average grades for the regular attendees and the absent students were 70.5% and 61.8%, respectively.

Assume that the population standard deviation for both groups is 12% and the grades followed a normal distribution.

Compute and interpret a 90% confidence interval for the mean difference in midterm exam grades.