A reversible Carnot engine has an efficiency of 30 percent and it operates between a heat source maintained at t1 and a refrigerator maintained at 210 k. what is the temperature of the heat source.
A Carnot engine has an efficiency which results from computation as equal to
$\eta = 1 -T2/T1$ where T2 is the absolute temperature of the refrigerator and T1 the absolute temperature of the heat source.
$T1 = T2/(1-\eta) =210 /(1-0.3) =210/0.7 =300 K$
The Celsius temperature of the heat source is
$t1 = T1-273 =300-273 = 27$ Celsius degree.