What subject is this?
Anyways the answer is:
MOST DEFINITELY!
For the first assessment of mathematics (had a weighting about 20% as well!), I was ranked 12th (maybe even further down haha). Anyways the most important thing is to never give up (as cheesy as it sounds). The first assessment is never an indicator of how well you'll perform in the HSC. I knew what I did wrong, and I worked on improving myself. You do not have to get full marks either. I was ranked 6th in half yearly, and the third term assessment was 15th. Full marks were only one or two in the grade. I ended up with first place in Trials and a state rank in mathematics ^^ (though our conditions may slightly vary, as my cohort did well in general). It just really shows that hard work does pay off, and that a state rank is very possible! In addition, it is very important that the person in front of you, in terms of ranking, also performs well in the HSC. That's also an important factor as both of you may end up with a state ranking! Best of luck with your studies.