The  vapour density of mixture having 64% by mass of CH₄ , 32% by mass of O₂ and remaining is H₂ is

Molecular weight = $\frac{\mathrm{Total} \mathrm{mass}}{\mathrm{Total} \mathrm{number} \mathrm{of} \mathrm{moles}}$

Total number of moles = $\frac{64}{16}+\frac{32}{32}+\frac{4}{2}=7$

Molecular weight of mixture = $\frac{100}{7}=14.28$

Vapour density of mixture = $\frac{14.28}{2}=7.14$