Calculate Kp at this temperature for the reaction PCl5(g) ↔ PCl3(g) + Cl2(g)?

A sample of gaseous PCl5 was introduced into an evacuated flask so that the pressure of pure PCl5 would be 0.4800 atm at 520 K. But, PCl5 decomposes to gaseous PCl3 and Cl2. At equilibrium the TOTAL pressure in the flask was measured at 0.8064 atm.