For $X{Y}_{2\left(g\right)}\rightleftharpoons X{Y}_{\left(g\right)}+Y_{\left(g\right)}$ , initial pressure of $X{Y}_2$ is 800 mm & equilibrium pressure is 960 mm. Degree of dissociation of $X{Y}_2{}_{\left(g\right)}$ is

  $X{Y}_{2\left(g\right)}\rightleftharpoons X{Y}_{\left(g\right)}+Y_{\left(g\right)}$

    $100$

    $\underset{\_}{xxx}$  

     $\left(1-x\right)xx$

$1mole800mm$

$\left(1+x\right)mole960mm$

         $\Rightarrow x=160mm$

$\therefore X=\frac{160}{800}=0.2$