If you go through the eligibility criteria then in the eligibility criteria they made  it clear that a candidate belonging to general OBC category must have 75% mark in 12th which include subjects physics chemistry mathematics one language and one any other subject therefore there is no such rule on top 20 percentile so to able to get admission into IISC you must score 75% in class 12th.

For more details on eligibility criteria please visit the link given below

https://www.google.com/amp/s/engineering.careers360.com/articles/iist-btech-admission/amp

hello shubham kumar,

may i know your percentage and category which you belong to.because iist admission need some requirements so please tell me this detaisl then only i can help you.

thank you!