Problem on molecular weight of solid

Question

The vapor pressure of pure benzene at a certain temperature is 200 mm Hg. At the same temperature the vapor pressure of a solution containing 2g of non-volatile non-electrolyte solid in 78g of benzene is 195 mm Hg. What is the molecular weight of solid:

(a) 50 (b) 70 (c) 85  (d) 80

Answer (d): (P^o - P_s)/P^o = n/(n + N)
(P^o - P_s)/P^o = (w x M)/ (m x W) = 80