I have taken a drop this year so that I can appear in JEE 2015. This year I scored 83% in CBSE boards. So I was not in the top 20 percentile. But as per new rule it is 75%. But is this rule applicable to students who have taken a drop?

There has been no formal statement about it. It just been said that 75% marks are required instead of 80 percentile.By the looks of it, it is applicable to the students who have taken the drop too.Otherwise, it sounds baseless if they are allowing for current students but not for past students.So most probably, it is applicable to you too. Just wait for the formal statement from IIT(next month probably), this is just a press statement letting know of the changes before hand. Further details will be out soon.