first we find the mean (average) by adding up all the numbers and dividing by how many numbers there are.
(21 + 10 + 15 + 19 + 8 + 17 + 15) / 7 = 105 / 7 = 15
now we find the absolute value of the difference between the data set and the mean
| 21 - 15 | = 6
|10 - 15 | = 5
|15 - 15 | = 0
|19 - 15 | = 4
|8 - 15 | = 7
| 17 - 15 | = 2
| 15 - 15 | = 0
now we find the mean (average) of these numbers
(6 + 5 + 0 + 4 + 7 + 2 + 0) / 7 = 24/7 = 3.43 <=== the MAD
