If I am failed in one subject but due to less marks in other subjects I can't score 75% can I give exam in other to subjects
Yes. You can appear for betterment exams to improve your percentage. But to complete Class 12 and be termed 'Pass' you need to pass in all subjects. So you have to appear for Supplementary exams in the subject you have failed and appear for betterment exams in the subjects you have passed to increase your overall aggregate.