Hello Justsane, 75% is aggregate marks means total marks divided by no of subjects. For JEE you should score 75% aggregate marks in 12 or should be in top 20 percentile of your respective board.

Top 20 percentile of any board for JEE is calculated by marks scored in five subjects which are

Physics

Chemistry

Mathematics

Language subject

Any subject other than physics, chemistry, Maths and language paper

If any body has taken 2 language paper then higher marks scored subject will be taken for calculating top 20 percentile.