If you only want to pass in HSC Physics and biology you need total 35 % marks.

20% of 70 marks theory + remaining practical marks = 40 marks

14 marks + remaining practical marks = 40 marks

but even if you have 25/30 in practical it still doesn’t mean that you doesn’t need 14 marks in theory.

So in total you need 15 marks to score in theory exam to get total 40/100 marks for both subjects