Hi pgburel,
From your post, it's not clear whether you're just beginning your studies or if you've spent some time preparing for the GMAT already. Have you taken any FULL-LENGTH practice CAT Tests yet? If not, then you can download 2 for free from
www.mba.com (and they come with some additional practice materials). If you're just beginning, and you want to do a little studying first so that you can familiarize yourself with the basic content and question types, then that's okay - but you shouldn't wait too long to take that initial CAT. That score will give us a good sense of your natural strengths and weaknesses and will help provide a basis for comparison as you continue to study. A FULL CAT takes about 4 hours to complete, so make sure that you've set aside enough time to take it in one sitting. Once you have those scores, you should report back here and we can talk through your study plan.
To answer your immediate question though, since the GMAT is an adaptive Test, questions can be designed for any difficulty level - meaning that there really aren't specific subjects that are exclusive to a particular score range. For example, on Test Day you could see a real straight-forward combinatorics question that isn't all that difficult to answer (so you shouldn't 'skip' that subject during your studies).
I'd like to know a bit more about your timeline and goals:
1) What is your goal score?
2) When are you planning to take the GMAT?
3) When are you planning to apply to Business School?
GMAT assassins aren't born, they're made,
Rich