A tank of volume $0\mathrm{.}3m^3$ is filled with air at $\frac{35}{b}ar,40C.$ The air discharges through a turbine into atmosphere until the tank pressure has fallen to the atmosphere pressure of $\frac{1}{b}ar.$ The turbine is applied to lift a mass of $500kg$ for $100m$ with a mechanical efficiency of $0\mathrm{.}85$ for the power transmission device. Please determine the isentropic efficiency of the turbine. $(20)$