Depends a bit on how many points the rest of the class was worth compared to the final. But for example's sake let's pretend it was two midterms also worth 100?
Since you say you are averaging 80% I'll just put 80s in for both midterms. So now we have:
80 + 80 + 0 = 160 / 300 which is a little over 50%, not good at all. But...
80 + 80 + 50 = 210/300 which is 70%. So in this scenario you only need a 50% on the third test to pass the class.
Does that help?