We have 70 marks for theory and 30 marks for practical in Physics and Chemistry. Those two components make up 100 marks which is the total marks in both the subjects. To pass, you have to score 33% in each because that's the passing criteria. So, 33% of 70 is around 23 and 33% of 30 is around 10. That makes a total of 33 out of 100. According to the stats you gave in your question, passing criteria for practical is satisfied but not for theory part. If you manage to score 20% marks out of 70, that means 14 marks, then in order to pass your total should be around 35 marks.
Hope this helped.