An engine using 1 mol of an ideal gas initially at 17.4 L and 285 K performs a cycle consisting of four steps: 1) an isothermal expansion at 285 K from 17.4 L to 36.1 L ; 2) cooling at constant volume to 122 K ; 3) an isothermal compression to its original volume of 17.4 L; and 4) heating at constant volume to its original temperature of 285 K . Find its efficiency. Assume that the heat capacity is 21 J/K and the universal gas constant is 0.08206 L · atm/mol/K = 8.314 J/mol/K. Give your answer in the form of a percentage.