\(\eta = 1 - \frac{{{T_2}}}{{{T_1}}}\,\,\,\,\,\,\therefore \,\,\,\frac{{25}}{{100}} = 1 - \frac{{300}}{{{T_1}}}\)
\(\therefore \,\,\,\frac{1}{4} = 1 - \frac{{300}}{{{T_1}}}\,\,\,\,\,\,\,\therefore \,\,\,\frac{{300}}{{{T_1}}} = 1 - \frac{1}{4} = \frac{3}{4}\)
\(\therefore \,\,{T_1} = 400\,K\,\,\,\,\,\therefore \,\,\,{T_1} = {127^ \circ }C\)