Hi prasad ,
As you are a ST category student you need to score atleast 70 marks to get a call from iims .as iims are one of the most sought after college in feild of management so they have high cutoffs for the candidates to get admission or get call for further interview process.

Hope it helps!!!

Dear student,
    To get topmost IIM's calls you need to attempt 70 questions with 85-90% of accuracy.
And to get a new IIM's calls you have to attempts 60-65 question with same accuracy.

If you able to maintain your accuracy joto this level
Your chances of getting IIM's calls will be very high

Good luck for your preparation