Hello Lakshmi,

While solving previous years question is a very good option to boost your score and get an edge over other students in the exam, it is close to sufficient but still it is not absolutely sufficient to get a very good score. Because the chances of repetition of questions are usually low and even if the question maybe based on the same pattern, still you need to have a good hold over the concepts of the subjects as it is very important and also a regular practice of model questions is a grade booster. Previous years questions are of course necessary condition for good marks but calling them sufficient would be unjust to the exam. 

All the best.