$0\mathrm{.}8$kg of air $($R$=294$J$/$kg$.$K and Cv $=711$J$/$kg$.$K$)$ is expanded according to the law PV$^1\mathrm{.}25=$ constant from initial conditions $1\mathrm{.}2$MPa and $0\mathrm{.}05$m$^3$ to a final pressure of $420$kPa. Determine the change in entropy and state whether it is an increase or decrease.