$P_i V_i=P_f V_f^\gamma$
$P_i(5)^{1.5}=P_f(4)^{1.5}$
$\frac{P_i}{P_f}=\left(\frac{4}{5}\right)^{\frac{3}{2}}=\frac{4}{5} \cdot\left(\frac{4}{5}\right)^{\frac{1}{2}} \Rightarrow \frac{8}{5 \sqrt{5}}$
$S1:$ The efficiency of a heat engine can be $1,$ but the coefficient of performance of a refrigerator can never be infinity.
$S2:$ The first law of thermodynamics is basically the principle of conservation of energy.
$S3:$ The second law of thermodynamics does not allow several phenomena consistent with the first law.
$S4:$ A process,whose only result is to transfer heat from a colder to a hotter object, is impossible.
