If the heat engine is a perfect carnot cycle engine, then
efficiency = (T2-T1)/T2
where T1=temperature of cold reservoir ( ° K) = 250+273.15 = 523.15
T2=temperature of hot reservoir ( ° K) = 715+273.15 = 988.15
Therefore
efficiency = (T2-T1)/T2 = (988.15-523.15)/988.15 = 0.4706 = 47%
Note: this is the theoretical maximum efficiency of a perfect heat engine. Real-life engines do not achieve this value.
