(a) $\eta = 1 - \frac{T_{2}}{T_{1}} = 1 - \frac{100}{500} = 1 - \frac{T}{900}$ $\therefore\ \ \frac{T}{900} = \frac{1}{5}\ \ or\ \ \ T = 180\ K$
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Ph.D. in Physics with 19+ years of teaching experience.