$2Fe_xO_y+(6x-2y)H_2SO_4\rightarrow xFe_2(SO_4)_3+(3x-2y)SO_2\uparrow+(3x-y)H_2O$$\Rightarrow \begin{cases}\frac{ax}{56x+16y}=\frac{2b}{400} \\0,0075x=\frac{b(3x-2y)}{400} \end{cases}\Rightarrow \begin{cases}a=\frac{0,015(56x+16y)}{3x-2y} \\ b=\frac{3x}{3x-2y} \end{cases}$
$*)\begin{cases}FeO \\a=1,08 gam\\b=3 gam \end{cases}$
$*)\begin{cases}Fe_3O_4 \\ a=3,48 gam\\b=9 gam \end{cases}$