Dear Soumya, for calculating top 20 percentile, the marks scored in Physics, Chemistry, Mathematics, a language and any other subject apart from the ones mentioned are taken into consideration. In case any candidate has selected 2 languages or 2 subjects, the one with higher score is considered. For more information, please visit our page at:
https://engineering.careers360.com/articles/top-20-percentile-cutoff-of-class-12-boards-for-iit-admission