The answer is 0.6670.667.
The combustion of acetylene can be written as
2C_2H_2 + 5O_2 -> 4CO_2 + 2H_2O2C2H2+5O2→4CO2+2H2O
The balanced chemical reaction shows a 2:52:5 mole ratio for C_2H_2C2H2 and O_2O2, a 1:21:2 ratio for C_2H_2C2H2 and CO_2CO2, and a 1:11:1 ratio for C_2H_2C2H2 and H_2OH2O.
Now, for simplicity, let us assume that we have 11 mol of C_2H_2C2H2 to start with ( the equivalence of 2626 grams ); given the fact that the oxygen is in excess, acetylene will always act as a limiting reagent.
So, one would get 22 moles of CO_2CO2 and 11 mole of H_2OH2O in the products, for a total of n_T = 2 + 1 = 3nT=2+1=3 moles of product.
Therefore, the mole fraction of CO_2CO2 would be
n_(CO_2) = (n_(CO_2)/n_T) = 2/3 = 0.667nCO2=(nCO2nT)=23=0.667
In this case, 20%20% excess O_2O2 would mean that you started with 33 moles of O_2O2 instead of 2.52.5, but I don't see how this would be relevant other than determining that O_2O2 is not a limiting reagent.