How to Find the Mean Absolute Deviation (MAD) for 2 Sets of Data: Test Scores

The problem:
Ms. Christensen gave two of her history classes a test. The following table shows the scores from her classes. Find the mean absolute deviation of each. If the MAD is similar for both, find the distance between the centers of the two classes.

The solution:
5th period MAD = 7
6th period MAD = 12

Distance between centers= 15.2