Hi jyotsnamahajan,
Permutations, Combinations and Overlapping Sets are all relatively rare categories on the GMAT. You'll probably see just one question of each on Test Day, although you could potentially see more based on how well you're performing (and the Experimental questions that you might receive). This is meant to say that none of those categories is worth too much of your time unless you're already doing well on the bigger categories (Algebra, Arithmetic, Data Sufficiency in general, etc.).
Before I can offer you any additional advice for your studies, it would help if you could provide a bit more information on how you've been studying and your goals:
Studies:
1) How long have you studied?
2) What study materials have you used so far?
3) How have you scored on EACH of your CATs (including the Quant and Verbal Scaled Scores for EACH)?
Goals:
4) What is your goal score?
5) When are you planning to take the GMAT?
6) When are you planning to apply to Business School?
7) What Schools are you planning to apply to?
GMAT assassins aren't born, they're made,
Rich