For the gaseous reaction $3H_2+N_2\rightleftharpoons 2N{H}_3,$ the equilibrium pressures of $H_2$  and $N_2$ is $0.4atm$ and $0.8atm$ respectively. The total pressure of the equilibrium system is 2.8 atm.  The value  of  $K_p$ is

$3H_2+N_2\rightleftharpoons 2N{H}_3$                                     $GivenP=2.8$

$0.40.8?$                                                    $P_{N{H}_3}=2.8-\left(P_{H_2}+P_{N_2}\right)$

                                                                                            $=2.8-1.2=1.6$

$K_p=\frac{1.6\times 1.6}{\left(0.8\right)\times {\left(0.4\right)}^3}$

       $=\frac{2\times 1.6}{0.4\times 0.4\times 0.4}$

       $=\frac{200}{4}=50at{m}^{-2}$