Convert 20 grams of H2O into moles by dividing 20 by 18. this is 1.11 moles
Now look and see if the equation is balanced. We can see it not. So balance this equation.
I believe you get 2C2H6 + 7O2 ---> 4CO2 + 6H2O
For every 6 moles of H2O you will get 2 moles of C2H6 based upon the balanced equation.
Now take the answer you got earlier
1.11 moles H2O/6 moles H2O * 2 moles 2C2H6 and then multiply it by the molecular weight of C2H6.