N₂O₄ is dissociated to 3 3% and 40% at total pressure P₁ and P₂ atm respectively. Then the ratio P₁/P₂ is:

$\underset{1-\mathrm{\alpha }}{\underset{1}{{\mathrm{N}}_2{\mathrm{O}}_4}}\rightleftharpoons \underset{2\mathrm{\alpha }}{\underset{0}{2{\mathrm{NO}}_2}}$
${\mathrm{K}}_{\mathrm{p}}=\frac{{\left({\mathrm{n}}_{{\mathrm{NO}}_2}\right)}^2}{{\mathrm{n}}_{{\mathrm{N}}_2{\mathrm{O}}_4}}\times {\left[\frac{\mathrm{P}}{\mathrm{\Sigma n}}\right]}^1$
For 33% dissociation ${\mathrm{K}}_{\mathrm{p}}=\frac{(2\times 0.33{)}^2}{0.67}\times \left[\frac{\mathrm{P}}{1.33}\right]$
For 40% dissociation ${\mathrm{K}}_{\mathrm{p}}=\frac{(2\times 0.40{)}^2}{0.60}\times \left[\frac{\mathrm{P}}{1.40}\right]$
$\therefore \frac{{\mathrm{P}}_1}{{\mathrm{P}}_2}=1.56\approx 1.60=\frac{8}{5}$