1. **State the problem:** A heat engine operates between a hot reservoir at temperature $T_H = 400$ K and a cold reservoir at temperature $T_C = 280$ K. The manufacturer claims the engine's efficiency is 35%. We need to determine if this claim is reasonable. 2. **Recall the formula for maximum efficiency:** The maximum possible efficiency of a heat engine operating between two reservoirs is given by the Carnot efficiency: $$\eta_{\text{Carnot}} = 1 - \frac{T_C}{T_H}$$ where temperatures are in Kelvin. 3. **Calculate the Carnot efficiency:** $$\eta_{\text{Carnot}} = 1 - \frac{280}{400} = 1 - 0.7 = 0.3 = 30\%$$ 4. **Compare the claimed efficiency to the Carnot efficiency:** The claimed efficiency is 35%, which is greater than the maximum theoretical efficiency of 30%. 5. **Conclusion:** Since no real engine can be more efficient than a Carnot engine operating between the same two temperatures, the claim of 35% efficiency is not reasonable. **Final answer:** No, the claim is not reasonable because it exceeds the Carnot efficiency limit.