Consider the following pattern of numbers :

$\begin{array}{cccc} 8 & 10 & 15 & 13 \\ 6 & 5 & 7 & 4 \\ \hline 4 & 6 & 8 & 8 \\ \hline 6 & 11 & 16 & ? \end{array}$

What is the number at ? in the above pattern?