Two reversible engines A (using helium) and B (using steam) both operate between 500 K and 300 K. (a) What is each engine's efficiency? (b) A student claims engine B is more efficient because steam is a better working fluid. Refute using Carnot's theorem. Solution: (a) Both: eta = 1 - 300/500 = 40%. (b) Carnot's theorem part 2: all reversible engines between the same reservoirs have identical efficiency regardless of working substance. The working fluid affects practical details, never the reversible limit.