Pure $O_2$ diffuses through an aperture in 224 seconds whereas mixture of $O_2$ and another gas containing $80\%\;O_2$ diffuses from the same in 234 sec. What is mol. wt. of gas?

Question

$(a)\;34.92\qquad(b)\;33.90\qquad(c)\;32\qquad(d)\;46.6$