For the equilibrium reaction N₂O₄$\rightleftharpoons$2NO₂ at certain temperature 20% of reactant is dissociated into 

products . If ‘p’ is total pressure then 

$$\begin{gathered} {\mathrm{N}}_2{\mathrm{O}}_4\rightleftharpoons 2{\mathrm{NO}}_2 \\ 1 0 ---\mathrm{initial} \mathrm{moles} \\ 1-\mathrm{x} 2\mathrm{x}---\mathrm{moles} \mathrm{at} \mathrm{equilibrium} \end{gathered}$$

total moles at equilibrium = 1 + x

${\mathrm{p}}_{{\mathrm{N}}_2{\mathrm{O}}_4}=\left(\frac{1-\mathrm{x}}{1+\mathrm{x}}\right)\mathrm{p}$

${\mathrm{p}}_{{\mathrm{NO}}_2}=\left(\frac{2\mathrm{x}}{1+\mathrm{x}}\right)\mathrm{p}$

Kp = $\frac{{\left({\mathrm{p}}_{{\mathrm{NO}}_2}\right)}^2}{\left({\mathrm{p}}_{{\mathrm{N}}_2{\mathrm{O}}_4}\right)}$

substituting x = 0.2 and solving 

p = 6 K_p