A gas refrigeration cycle with a pressure ratio of 4 uses helium as the working fluid. The temperature of the helium is -6C at the compressor inlet and 50C at the turbine inlet. Assuming isentropic efficiencies of 0.85 for both turbine and the compressor, determine (a) the minimum temperature in the cycle, (b) the coefficient of performance, and (c) the mass flow rate of the helium for a refrigeration rate of 25kW.