$3H_2+N_2\rightleftharpoons 2NH_3$ $3x x 2x$
$\Rightarrow\begin{cases}P_1 \\ n_1=(0,4-x)+(1,6-3x)+2x=2-2x \end{cases}$
$2NH_3+3H_2SO_4\rightarrow N_2+3SO_2+6H_2O$
$2x x 3x 6x$
$\Rightarrow \begin{cases}P_2 \\n_2=(0,4-x)+(1,6-3x)+(x+3x+6x)=2+6x \end{cases}$
$\Rightarrow \frac{P_1}{P_2}=\frac{2-2x}{2+6x}=\frac{1}{1,75}\Rightarrow x=\frac{0,75}{4,75}\Rightarrow h=\frac{0,75.100}{4,75.0,4}=39,47\%$