Hello dear

With this percentile ( 83 ) it is not possible to get 15500 rank .

And talking about fee concession ; it is based on the caste ; category or state not on basis of jee rank / score in good colleges.

Also based on the family income .

In nits 100% fee concession if income is less than 1 lakh

2/3 of fee concession more than 1 lakh 

1/3 for 2.5-5 lakhs.

No fee concession if greater than 5 lakh 

(The provided figures may vary ).

So good luck.