Final answer:
The cold reservoir at 168°C temperature the Carnot engine has an efficiency of 60%.
Step-by-step explanation:
To find the cold-reservoir temperature at which a Carnot engine with a hot-reservoir temperature of 420°C has an efficiency of 60%, we can use the formula for the efficiency of a Carnot engine:
Efficiency = 1 - (Tc/Th)
Where Tc is the temperature of the cold reservoir and Th is the temperature of the hot reservoir.
Given that the efficiency is 60% and the hot-reservoir temperature is 420°C, we can rearrange the formula to solve for Tc:
Tc = Th * (1 - Efficiency)
Substituting the values, we have:
Tc = 420°C * (1 - 0.60)
Tc = 420°C * 0.40
Tc = 168°C
Therefore, the cold-reservoir temperature at which the Carnot engine has an efficiency of 60% is 168°C.