Sadly, it's not possible for anybody (including, I suspect, the test-writers themselves) to give you an accurate, simple breakdown of the question distribution. Many of the questions on the GMAT are difficult to categorize, since a single question can cover several topics (it isn't unusual to see probability mixed with number properties or geometry mixed with ratios, for example). Even if we could categorize questions perfectly, it seems that the question composition differs substantially from test to test, partly because there seems to be some unpredictability built into the GMAT algorithm, and partly because you'll see some experimental questions that could come from any category.
So the useless answer is that you should be prepared for anything.
If you absolutely have to skip some topics, then I would probably spend less time on rates, combinations and permutations, probability, and overlapping sets. For each of these four categories of questions, you probably will only see one or two questions, and you might see zero questions from one of the categories if the GMAT gods are in the right mood. Then again, I've heard of GMAT test-takers who saw as many as five probability questions on a single test. That's extremely rare... but if you have to cut something out of your studies, these four categories would probably be the best areas in which to cut corners. I would recommend understanding the most basic variations in each of these topics so that you don't miss easy questions, but if your score goals are relatively modest, you probably don't need to master the hardest concepts in these categories.
The topics that are important, in my eyes: algebra (including inequalities, absolute values, exponents, certain word problems), arithmetic (including ratios, percents, roots, exponents, and more word problems), and number properties (factors, multiples, primes, remainders, etc.). About 2/3 of your questions will come from these three categories. You can usually expect to see somewhere around 5 geometry questions, so you probably want to spend at least some energy there, and it's usually worthwhile to spend a few minutes making sure that you understand the GMAT's favorite statistical terms (mean, median, range, standard deviation). Get as much exposure as you can to official GMAT questions so that you have a good sense of the GMAT's funny way of phrasing questions and combining concepts.
Sorry, I know that this is an unsatisfying answer, but the GMAT is a strange beast. Some people don't need to study much at all to achieve their score goals, but if you're like most of us, you'll find that the test really isn't friendly to test-takers who have limited time on their hands.
I hope this helps a little bit. Good luck with your studies!
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