Answer:
2.57
Explanation:
The mean absolute deviation is the mean of the absolute values of the differences from the mean.
Application
The mean of the dataset is the sum of values, divided by their number:
mean = (20 +16 +21 +16 +22 +16 +15)/7 = 126/7 = 18
The absolute values of the differences from 18 will have an average of ...
mean(|differences|) = (2 +2 +3 +2 +4 +2 +3)/7 = 18/7
mad(data) ≈ 2.57
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Additional comment
Some calculators have the mad( ) function built-in. See the second attachment, for example.