$Mg+H_2SO_4\rightarrow MgSO_4+H_2\uparrow$$x x x x$
$Zn+H_2SO_4\rightarrow ZnSO_4+H_2\uparrow$
$y y y y$
$\Rightarrow n_{H_2SO_4}=x+y\Rightarrow m_{dd}=\frac{98(x+y).100}{20}+24x+65y-2(x+y)$
$\Rightarrow m_{dd}=512x+553y$
$\Rightarrow \frac{120x}{512x+553y}=\frac{15,22}{100}\Rightarrow x=2y$
$\Rightarrow \%ZnSO_4=\frac{161y}{512x+553y}.100=10,21\%$