To solve numerical given in the exercise of the NCERT text one must have a very clear understanding of the chapter and concepts. Read the chapter from the textbook and try to understand the concepts.
Make your notes of all important formulas in a separate sheet and revise it thoroughly. start with some solve example and try to build the approach and learn how to solve the numerical questions on how to use the formulas what the questions and concepts are used in the questions. After that attempt, the exercise questions are given in the NCERT textbook.