Top 20 percentile cutoff marks for 2020 has not been declared yet. You can check last few years' top 20 percentile cutoff (out of 500) as reference -
Bihar School Examination Board, Patna:
For 2019
OC- 346 ; OBC- 347 ; SC- 332 ; ST- 339 ; PWD- 332
For 2018
OC- 293 ; OBC- 287 ; SC- 279 ; ST- 284 ; PWD- 279
For 2017
OC- 317 ; OBC- 312 ; SC- 302 ; ST- 306 ; PWD- 302
You can also check this link- https://josaa.nic.in/webinfocms/Handler/FileHandler.ashx?i=File&ii=163&iii=Y
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