Haariskhan

I'm really stuck on this question: A fuel gas consists of methane and ethane in unknown proportions is burned in a furnace with air. Combustion is known to be complete and an analysis of the flue gas on a dry basis gives 85.99% N2 and 10.88% CO2. Determine a) composition of the fuel gas, b) the percentage excess air used. My basis I've chosen(Don't know if this is an appropriate basis) is 1 kmol of the fuel gas. I've let x=kmol CH4 and 1-x=kmol C2H6. From there I wrote equations for CO2 formed and theoretical O2 needed, but now I am extremely stuck. Can someone give me a method of approaching these sorts of questions? I don't even know if what I have done above is right.