Question

Consider a Carnot engine operating between the temperatures of 300. K and 189 K. Find the efficiency of this engine. Assume, now, that the isothermal step at 300. K is not reversible and the expansion takes place against a constant external pressure of 5.00 atm. Find the efficiency of the engine under these conditions. Take:
P1= 10.0 atm, V1= 1.0L T(H)= 300.K
P2= 5.0 atm, V2= 2.0 L, T(H)= 300. K
P3= 1.57 atm, V3= 4.0L, T(L= 189 K
P4= 3.14 atm, V4= 2.0L, T(L)= 189. K
Note that n is not equal to 1 and Cv= 3/2 R

I found the first efficiency. I'm working on the second set of conditions. I know four equations (one for each step of the carnot cycle) that, when added up, find the total work. Could I plug this into Efficiency= -w/q(H)? And if so, how do I find q(h)? Or am I going about this the wrong way?

Also, can I plug in PV/RT for n, since the number of moles is unknown?