Relax. 20days are sufficient to score decent like 80+ in board exam. I’m a science student. So assuming that you score 25+ in Physics and Chem practical out of 30 and 15+ in maths practical, if you just score 55+ in Phy and Chem out of 70 and 65+ in maths out of 80, you can easily cross 80%. Now, scoring these marks is not at all a big deal.
Make a plan like 7days pet subject. Try to read your textbooks at least 2times in this period, write down all the derivations as many times as you can. Make your concepts clear and you can easily do it. Try to learn the definitions from textbook. And do the question given in the exercise only as we don’t have that much time. But yes, you can manage to score well.
Wish you all the best!