Round the clock cooling of an apartment having a load of $$300$$ $$MJ/day$$ requires and air-conditioning plant of capacity about

During the phase change of a pure substance:

Which among the following relations is/are valid ONLY for reversible process undergone by a pure substance?

Figure below shows a reversible heat engine $${E_R}$$ having heat interactions with three constant temperature systems. Calculate the thermal efficiency of the heat engine.