Hello,
I think that the median (average) score is 30, not 25 (and different numbers of people get each score, so different numbers of people get 28, 29, 30, 31, 32 and so forth).
Generally, the score with the highest number of people getting it is 30 and either side of 30, fewer people get each score, decreasing the further you move away from 30 (so fewer people get 20 than 25, and fewer people get 40 than 35).
The VCAA website provides some information about this process, if you are interested:
http://www.vcaa.vic.edu.au/faqs/vcecurrentstudents.htmlAlso, not every C+ is equal, it's more your actual mark on the exam (e.g. 40/72, 36/72, 44/72 for the chem exam) that determines your study score. C+ is just a more broad way of telling you where you stand.
So if you get C+ on both your exams, then you have a reasonable chance of getting a 30 (since that is the average). Your SACS also get taken into account (the median there is a bit higher than C+), but if your score is around the median for your exams and SACS, then there's a decent chance of getting a 30.
From the VCAA website: •78% of students will get a score on or above 25
As a rough guess (I'm no expert here), I'd say a low C is required in the exam to get 25.