Hi AlN,
In general, Permutations, Combinations and Probability questions are typically considered harder-than-average because each of those question types is a subject that most people never learned while they were in school (and those questions often require a specific formula or logic to solve). That does NOT mean that every question in those 3 categories is difficult though - and you might see some fairly 'simple' versions of those questions on Test Day. From a frequency-standpoint, none of them are all that common on Test Day either - you will likely see 1 of each; they might be difficult, but they might not. The "hard" questions that you see on Test Day could just as easily be based on other subjects, such as multi-shape Geometry, advanced Algebra, etc.
When it comes to averages, you will almost certainly see at least 1 question that refers to the standard 'average formula' and another that refers to 'average speed.'
Before I can offer you any additional advice, 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 besides the books that you mentioned?
3) How have you scored on EACH of your CATs/mocks (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