Can you show how you got 80%.
Oops! I had a mistake.. I forgot to convert moles to the concentration. Now I get 66% the same as you. Since K=2 and the concentration of water and ethylene is 1 mol/lit , the concentration of ethanol would be 2 mol/lit. If the reaction were to be completed then the conc. would be 3 mol/lit. Thus the yield of the reaction is 2/3 = 66%!
So what's wrong with the question?!