An engine using 1 mol of an ideal gas initially at 23.9 L and 394 K performs a cycle consisting of four steps: 1) an isothermal expansion at 394 K from 23.9 L to 41.1 L; 2) cooling at constant volume to 277 K; 3) an isothermal compression to its original volume of 23.9 L; and 4) heating at constant volume to its original temperature of 394 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.