To get into the most popular business school in our country, you must first appear in CAT for IIMs and for other institutes while XAT for XLRI. To get a call, you must get at least 70 percentile in each section and more than 80 percentile on the CAT.
CAT cutoff goes up to 99.8, and a general candidate with an 80 percentile score has a very small chance of getting in. If you have a bachelor's degree in medicine, law, or the arts and got at least 90 in 10th and 12th grade, more than 75 in graduation, or were in the top 10% of your field, IIM A may call you around 85 percentile.
If you want to get into B.Com, you need 96+ and the above-mentioned academics. Engineering graduates who appear in the CAT get the best score in the stream from which they have majored. There is a very small chance of getting a call below 99. For an engineering graduate, getting a call from IIMA means scoring 99.5+ on the CAT, getting 90+ in 10th and 12th grade, and getting 85+ in college.
Last, I would say that IIM Ahmedabad gives a lot of weight to academic diversity and academic success in the past. So, if you didn't do well in school or college, you won't get a call from IIM A. Don't let that get you down. Instead, study hard for the CAT and do your best. One can still get into schools like IIM C, IIM K, FMS, and MDI, which give CAT scores a lot more weight. There are many other good B-schools that you can get into with 90/95+ percentile.
Other than CAT score your GDPI performance, previous academics, diversity, and work experience also play a role in the final selection.