$Mg+2HCl\rightarrow MgCl_2+H_2\uparrow$$0,15 0,15$
$2Mg+O_2\rightarrow 2MgO$
$x x$
$2Cu+O_2\rightarrow 2CuO$
$y y$
$\Rightarrow \begin{cases}24x+64y=m \\40x+80y=1,25m+a \end{cases}\Rightarrow 1,25(24x+64y)-6x=1,25m+a$
$\Rightarrow -6x=a\Rightarrow \begin{cases}x=0 \\ y=0 \end{cases}\Rightarrow C_{M(HCl)}=\frac{0,15}{0,2}=0,75 M$