For JEE exams total aggregate in five subject i.e Physics, Chemistry, Maths, A language, Any other subject should be more than 75%  overall percentage and for nit also Candidates should satisfy the following JEE Advanced eligibility criteria for admission They must have secured at least 75% aggregate marks in the Class 12 (or equivalent) Board examination.

Your overall percentage of 83% in Class 12th is sufficient to meet the basic eligibility criteria for appearing in JEE Mains. The specific requirement of 60% in each subject is incorrect.

However, to secure admission into an NIT, you need to fulfill two primary conditions:

1. Qualify for JEE Mains: While there's no minimum percentage requirement for appearing in JEE Mains, a good score is essential for securing a good rank.
2. Meet the NIT Admission Criteria: Most NITs require candidates to have secured at least 75% marks in their Class 12th board exams (or be within the top 20 percentile) for admission. There are relaxations for reserved categories.
   
   

Your 55 marks in Maths might not pose an issue as the overall percentage is above 75%. However, it's crucial to focus on your JEE Mains preparation to secure a good rank, as competition for NIT seats is high.

Remember to check the specific eligibility criteria of the NITs you're targeting as there might be slight variations.