I have 80% in 12,am i eligible for jee, BITSAT after giving improvement in chem. And maths only or do i need to give in all subjects.

Hey Arnav!
If you have 80% in class twelfth then you do not need to worry at all!
You are surely eligible for both Jee Mains and Bitsat.
For BITSAT, you need to have a minimum of 75% in PCM and 60% individually in P, C, M.
In Jee Mains, you must only have passed all the exams.
Also, even if you are appearing for improvement, you are still eligible for the exams!

You only need to give improvement exams for the subjects where you did not pass. Your overall percentage should be 75% which you have but also for BITSAT you should have the percentage in PCM more than 60%. As long as these criteria are satisfied you can appear for BITSAT. As for JEE you are eligible already!