for admission into nit and iiit we have to satisfy both the criteria of percentage more than 75 % and came into top 20 percentile of state otherwise only one

Hello there,
The eligibility criteria for admission in IITs and NITs say that you have to get atleast 75% in your 10+2 level from a recognised board. This criteria should be fulfilled.
Coming to the next point, you need to get the cutoff rank required. Some seats are reserved under Home State Quota. You need to achieve that rank in JEE Mains and Advanced to get admission into that college for your desired course. This cut-off rank changes every year depending on the difficulty of the paper, and the number of students appearing for it. So there is no fixed cut-off rank.
Hope this answer helps you.
Good luck!!