Say you come second last in your cohort with, 55%? Then you go to the external exam (HSC) and top the cohort with 95%, your mark is whoever ranked second last in that external exam, plus the 95% you got. Let's say the person with the second last mark in the HSC exam got 40%...your mark is the average of both 40% and your 95%. This is a rare case, that you can rank that poorly then pull out a magic wand from behind and top the HSC, but it is just an example so you get a jist of what happens. I have seen so many forums that address this topic, it isn't funny. Study Hard, then you won't have this problem, trust me.