The temperature of source and sink of a heat engine are 127⁰C and 27⁰C, respectively. An inventor claims its efficiency to be 26%, then 

Efficiency of heat engine,   $\eta =1-\frac{T_2}{T_1}\text{ or }\eta =\frac{T_1-T_2}{T_1}$

Here,    $T_1=273+127=400\mathrm{K}$

$T_2=273+27=300\mathrm{K}$

$\therefore \eta =\frac{400-300}{400}=\frac{100}{400}=0.25=25\mathrm{\%}$

Hence, 26% efficiency is impossible for the given heat engine.