Dear Trupt Parkar,
Actually there are two Exams in this year''s IIT-JEE. IIT-JEE 2013 will have a IIT-JEE Main Paper which is like a qualifying exam for the IIT-JEE Advanced Paper.
For Selection into the NIT''s it is enough to give just the IIT-JEE(Mains) but if you are aiming for an IIT then you must give the IIT-JEE Advanced Paper which can only be given if you clear the Main paper.
To write the IIT-JEE Main paper you must be within the Top 20 percent of students in the Board Exams of your state in that Year. The Board exams carry a weightage of 40% and the score of IIT-JEE Mains carry a weightage of 60% and both determine your eligibilty to join a NIT and for qualifying for the IIT-JEE Advanced Paper.
Hope this helped you immensely..!!
All the Very Best & Good Luck to you ...!
Best Regards,
AskIITians Expert,
Godfrey Classic Prince
IIT-M
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