The mean deviation about mean of data 5,6,7,8,6,9,13,12,15 is

$x_i=\begin{array}{ccccccccc}5& 6& 7& 8& 6& 9& 13& 12& 15\end{array}$

n=9

$\overline{x}=\frac{1}{n}\sum xi=\frac{81}{9}=9$

Mean deviation about mean

$=\frac{1}{n}\sum |xi-\overline{x}|$

$=\frac{1}{9}[4+3+2+1+3+4+3+6]$

$=\frac{26}{9}=2.88$