The mean of six numbers. $x-5,\; x-1,\;x,\; x+2,\; x+4$ and $x+12$ is $15.$ Find the mean of first 4 numbers.

Question

$\begin{array}{1 1} (a)\; 9 \\ (b)\; 12 \\ (c)\;13 \\ (d)\;12.5 \end{array}$