if I got an in 1 st attempt nit can I give jee mains and jee advance again??

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Hello student if you get selected in any NIT's in first attempt but you're not satisfied you want to drop a year to get admission in a better college sure you can but you need to be clear with the eligibility criteria.

You can appear in two and three consecutive years in Jee advance and Jee mains respectively.
Eligibility - Candidates must passed or appearing class 12 with physics chemistry and mathematics with minimum 75 percent marks aggregate from a recognized board.

Feel free to comment if you have any doubt
Good luck

Yes you can give Jee mains & advance if you want to improve your score & get into a better college .