The Efficiency ($\eta$) of a perfectly reversible Carnot engine depends only on the temperatures of the hot source ($T_s$) and cold sink ($T_k$): $\eta =1-\frac{T_k}{T_s}$

• Given $T_s=400\text{ K}$ and $T_k=300\text{ K}$. $\eta =1-\frac{300}{400}=1-0.75=0.25$
• As a percentage: $0.25\times 100\%=25\%$.
• This represents the maximum possible efficiency that any heat engine can achieve between these two thermal reservoirs.