The mean absolute deviation is the mean of the absolute values of the differences between the given numbers and their mean. The mean is
... (79+85+90)%/3 = 84 2/3%
so the deviations are
... -5 2/3%, +1/3%, 5 1/3%
and the mean of their absolute values is
... (5 2/3 + 1/3 + 5 1/3)%/3 = 3 7/9%
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I find it convenient to let a calculator handle the tedious math.
