I figured it out. You're right, I did miscalculate one of the Q's. At the beginning, I calculated QH (or Q23) to be 303J but I know now that was where I went wrong, so I managed to get the right answer now. Thanks for all your trouble :)
Homework Statement
The figure shows the cycle for a heat engine that uses a gas having gamma =1.25. The initial temperature is T1=300K, and this engine operates at 20 cycles per second. What is the engine's thermal efficiency...