Help with Kp question

Oct 2018
1
0
England
Can anyone help me with this question? I have calculated the equilibrium moles and mole fractions and also written an equation for kp but don’t know what to do next.
“in the following reaction 4.0 moles of PCl5 is heated. At equilibrium there are 2.7 moles of PCl5 remaining. Given that the value of kp at this temperature is 3.68atm, what must the total pressure have been in the reaction?”