The efficiency of a heat engine can be calculated as the ratio of the work obtained to the heat energy put in at the high temperature,
efficiency = Work_max/Thigh.
or
efficiency=TH-TL/TH,
where TH is the high temperature and TL is the low temperature.
In our case the heat engine operates between 600K and 800K. SO, TH=800K and TL=600K.
The efficiency is: e=800-600/800=200/800=0,25
