Ans: $6$
$\begin{array}{rcl}
\dfrac{6\times4+9\times7+12\times10+15\times x}{4+7+10+x} & = & 11 \\
207 + 15x & = & 231 + 11x \\
4x & = & 24 \\
x & = & 6
\end{array}$
$\begin{array}{rcl}
\dfrac{6\times4+9\times7+12\times10+15\times x}{4+7+10+x} & = & 11 \\
207 + 15x & = & 231 + 11x \\
4x & = & 24 \\
x & = & 6
\end{array}$
