$A$ frictionless heat engine can be $100 \%$ efficient only if its exhaust temperature is ............

In a Carnot engine,the work done by the working substance is equivalent to:

Suppose that two heat engines are connected in series,such that the heat released by the first engine is used as the heat absorbed by the second engine,as shown in the figure. The efficiencies of the engines are $\epsilon_1$ and $\epsilon_2$,respectively. The net efficiency of the combination is given by:

$A$ reversible heat engine converts one-fourth of the heat input into work. When the temperature of the sink is reduced by $52 \, K$,its efficiency is doubled. The temperature in Kelvin of the source will be ...... .

$A$ Carnot's engine has an efficiency of $25 \%$ when its sink is at $27^{\circ} C$. If it has to be increased to $40 \%$,what should be the temperature of the sink keeping the temperature of the source constant (in $K$)?

Consider the given series combination of Carnot cycles. If $W_1 = W_2$,then the value of $T$ is ...... $K$ (all temperatures are maintained at their respective values).