The following examination scores, arranged in ascending order, were achieved by 20 students enrolled in a decision analysis course: 39, 46, 57, 65, 70, 72, 72, 75, 77, 79, 81, 81, 84, 84, 84, 87, 93, 94, 97, 97. Determine the (a) mean, (b) median, (c) mode, and (d) skewness for these scores.