We have, 17, 20, 12, 13, 15, 16, 12, 18, 15, 19, 12, 11

Mean of the given data is

The respective absolute values of the deviations from the mean , i.e. are

2, 5, 3, 2, 0, 1, 3, 3, 0, 4, 3, 4

Thus, the required mean deviation about the mean is