I worried about these problems way too much when studying for the GMAT.
I got a 43 in quant and remember getting 1 very simple combinatorics question (something like how many sandwiches can you make with 4 different ingredients)
Then I took the GMAT again and got a 47 in quant. I remember getting one simple probability question and one difficult combinatorics problem (something about arranging uppercase and lowercase letters)
But really, if you see a combinatorics/probability question on the test it's a sign that your doing well. And in my personal opinion, it's unlikely that you'll see more than 1-2 of these on the test, unless maybe you are doing outstanding.
My advice is to worry about more fundamental stuff on the exam. The first time I took the GMAT I was a combinatorics/permutation/probability expert. However, I think that this expertise came at the expense of doing poorly on some fundamental items. Unless you have a superb grasp of the fundamentals, your probably not going to get any difficult C/P/P problems anyways. So don't waste too much time on this.
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