What is the thermal efficiency of the cycle?

Question

An open cycle constant pressure gas turbine uses a fuel of calorific value 40,000 kJ/kg, with air fuel ratio of 80 : 1 and develops a net output of 80 kJ/kg of air. What is the thermal efficiency of the cycle?                                            

a. 61%                         b. 16%

c. 18%                         d. None