A gaseous fuel mixture stored at 745mmHg and 298 K contains only methane CH4 and propane C3H8. When 11.5L of this fuel mixture is burned, it produces 774kJ of heat. What is the mole fraction of methane in the mixture? (Assume that the water produced by the combustion is in the gaseous state.) I've tried 27.3%, 67%, and 40.7% and they're wrong.