The temperature-entropy diagram of a reversible engine cycle is given in fig. Its efficiency is

We know that change in entropy ∆S is given by ΔS=ΔQT     or     ΔQ=TΔSFor calculation of Q1 and Q2, see fig.Here,  Q1=T0S0+12T0S0=32T0S0Q2=T0S0 and Q3=0Now, η=WQ1=Q1−Q2Q1=1−Q2Q1=1−23=13