$m_{MgCl_2}=142,5 gam\Rightarrow \begin{cases}m_{dd MgCl_2}=\frac{142,5.100}{60}=237,5 gam \\ m_{dd HCl}=\frac{142,5.2.36,5.100}{95.40}=266,25 gam \end{cases}$$\Rightarrow m+m_{dd HCl}=m_{hh khi}+m_{dd MgCl_2}$
$\Rightarrow m=\frac{6,72.(18.2)}{22,4}+266,25-237,5=39,55 gam$