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If you haven't scored 75 percentage this year(2019) then you need to appear for improvement examination to be held next year in 2020. If you can score an aggregate of at least 75 percentage in your improvement examination then you can get admission in NITs, IIITs and IITs. You can appear for JEE Mains three times and JEE Advanced only two times. So, if your first attempt for JEE Advanced was in year 2019, then your next attempt would be in year 2020. And if your first attempt for JEE Mains was in year 2019, then you can further attempt JEE Mains in year 2020 and 2021 only. Please note that you can attempt for JEE Advanced only after qualifying the cutoffs of JEE Mains.