\(P_1 V_1=P_2 V_2\) [for isothermal]
\(P V=P \times 2 V\)
\(\frac{P}{2}=P^{\prime}\)
\(P_1 V_1^\gamma=P_2 V_2^\gamma\) [for adiabatic]
\(\frac{P}{2} \times(2 V)^{5 / 3}=P_2(V)^{5 / 3}\) \([\gamma\) for neon \(=5 / 3]\)
\(P=P_2 \cdot(2)^{-2 / 3}\)
Fractional decrease \(=\frac{P_2-P}{P_2}=\frac{P_2-P_2 \cdot(2)^{-2 / 3}}{P_2}=1-2^{-2 / 3}\)