We know,
molar mass=vapor density×2
molar mass = 27.6×2=55.2 g/mol
Molar mass of the mixture = mole fraction of NO2×molar mass of NO2+mole fraction of N2O4×molar mass of N2O4
Let 1 mole of mixture has x mole N2O4
2×27.6=x(92)+(1−x)46
⇒55.2=92x+46−46x
⇒x=0.2
Hence 0.2 mole of N2O4 is present in 1 mole of the mixture. Hence 2 mole of N2O4 will be present in 10 moles of the mixture.