Hello aspirant,
I am here for helping you....
As per your doubt  I am clarifying you that
Percentile indicates how many students have been outperformed by you in exam.Top 20 percentile means that you are ahead of 80% of the people. As for nit and iit either you have to be in top 20 percentile of your board or to have 75% in 12 board.
Now as per your doubts I am providing you some latest data from JOSAA counselling:
JOSAA category Wise Top 20 percentile JEE CUT-OFF marks for 2019:
Bihar Intermediate Education Council
GEN-313
OBC-NCL -307
SC-300
ST-303
PWD-300.
i hope it will be beneficial for you......
For more queries you can upvote my answer and can ask in the comment section below.....
Best of luck......

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