a mixture of C^₂H^₂ and C₃H₈ including sufficient amount of O₂ occupied a certain volume at 80 mm of hg .the mixture was burnt to CO₂ and H₂O(l).when the pressure of CO₂ was found to be 42.5 mm of hg at the same temp and volume find  the fraction of C₂H₂   the mixture.