Krishanu, as you have not specified your stream, so I am answering your query by assuming it to be Science with PCM subjects along with some additional subjects. See, it is very important to go through the previous years and sample question papers and analyse them to know about the important topics and the pattern of exam and you can't just rely on the NCERT exercise questions of all the chapters to get you 60+ marks out of 70 in the Board exams, especially in case of Physics and Maths. Although, the NCERT theory and the exercise questions are sufficient for the Chemistry exam to get you above 60 marks, in case your theory is strong and you have understood all the concepts and basics including the reactions, mechanisms and conversions properly but it is not the same in case of Physics and Maths and you should ideally go through the previous year papers to get an idea of the pattern of the paper and the type of questions asked from a topic. Also, many questions are framed directly from the application of theory in the NCERT textbooks as evident from the previous few years papers, so just solving the exercises and the important questions would not suffice. Yes, it may help you get a few questions right but if you are aiming for 60+ marks in theory, you would have to be thorough with the NCERT Theory and should have at least read it 2-3 times. If you are thorough with NCERT textbook and have understood its concepts, then the sample papers or the previous 10 years papers should be sufficient to get you 65+ marks as the exam isn't as tough as it seems but it requires the student to be clear about the concepts and theory and also its applications and the sample and previous year papers help to understand the theory and its' related questions in a proper manner which is important for performing well in the exams.