Writing the given observation in ascending order, we get 10, 13, 15, 17, 18, 19, 21. Given number of observations are 7.
`therefore n = 7` (odd)
`therefore "Median", bar(x) = 17`
`therefore` Mean Deviation
`= (|10-17|+|13-17|+|15-17|+|17-17|+|18-17|+|19-17|+|21-17|)/(7)`
`= (7+4+2+0+1+2+4)/(7) = (20)/(7) = 2.857`.