It is very easy to score more than 90 in Maths in Board exams since patter is fixed You can easily find out which topic is important and just look at the previous years' paper  You will know which questions are important, learn how to solve them. There is no need to solve tough questions from big books. If you are good at writing and framing answers in Your word then its GOOD otherwise learn How to write answers properly in maths. Also, focus on time management because maths paper is lengthy.