Hi Sandilya,
You would need to score at least 50% marks in your graduation (B.Tech in your case) to be eligible to apply for CAT exam. In the final year of your graduation, you can apply for the exam and simultaneously, you can start your preparations for the CAT exam. Just make sure that you start preparing for CAT at least 6 months before the exam date dedicatedly. Being average doesn't mean that you cannot crack the CAT exam. Scoring well in CAT is no rocket science. It's just regular practice, learning time-management skills and getting a professional guidance.
Get all the CAT preparation tips here (including tips from toppers and academic experts) - Tips for CAT Preparation
Hope that's helpful. All the best.
Candidates can check B.Tech Cut off for Top IITs & NITs here.
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