Hi pavel1200,
Most Test Takers are not great at Permutations, Combinations and Probability questions when they first begin their studies (since these subjects are typically not a part of any math classes that you would have taken previously). Thankfully, these categories are rarer on Test Day than the BIG categories (algebra, arithmetic, math formulas/rules, geometry, etc.) - that's why these 3 categories show up near the end of most GMAT Courses/Books.
Before I make any recommendations for how to learn/improve in these areas, I'm curious about how you're performing on the Quant section OVERALL? I ask because many Test Takers put too much emphasis on the rarer categories when they're not great at the big categories yet. As a measure for comparison, you'll likely see 15 or so questions that are based on specific Algebra rules (and you'll likely use Algebra "skills" on a number of other questions), but you'll probably see just 1-3 probability questions on Test Day. If you're not really "nailing" the big categories, then focusing on the rarer ones isn't worth your time.
1) How long have you been studying?
2) What is your score goal?
3) How have you been scoring on your practice CATs (including the Quant and Verbal Scaled Scores)?
GMAT assassins aren't born, they're made,
Rich