Correct answer: 800 K
The efficiency of a Carnot engine is given by η = 1 - (T_c/T_h), where T_c is the exhaust temperature and T_h is the intake temperature. For the first case, 0.4 = 1 - (T_c/500), solving gives T_c = 300 K. For the second case, 0.5 = 1 - (300/T_h), solving gives T_h = 600 K. Thus, the answer is 800 K.