Hi, kn2130:
Josh gives some great suggestions on how to work through the days leading up to the test, and I'd also like to highlight his comments on practice tests - just doing practice tests isn't how you improve...you have to take the time to analyze the results and determine how you can best improve from them in order to see any marked improvement in your scores!
The main reason I wrote, though, is in response to your subject line "How important is combinatorics":
A few years ago, when the testing of Permutations/Combinations was around its peak, most students (at least from my perception) saw Combinatorics as the type of question you would only see if you were doing really well. I never knew exactly how to respond to that, as I'm always leery of terms like "only", "always", and "never" when it comes to situations like these. I had to convince one student that he should at least go through the lesson when we went through it in class (he thought he might just study other subjects on his own with that time) and do 20 or so homework questions to get a feel for it.
When he emailed back with his score - I can't recall the number, but he was pretty pleased...something like high 600s when his goal was probably around 650 - he made special note to say that his first question was a permutation, and that another question he saw in the first 5 or so was, also.
So, I'd advise this - don't try to master combinatorics in a short period of time, for the reasons that Josh mentions. However, if you can at least get comfortable enough with the fundamentals (multiply the number of possibilities for each position in a basic permutation, etc.) to feel like you can tackle the 500-600-level permutations questions, you won't leave yourself open to the panic that might hit if you saw 1-2 combinatorics questions in the first dozen or so questions.
Combinatorics questions are pretty neat - while for the harder ones, it seems like you have to really understand them, a lot of the questions in the GMAT mix are fairly manageable if you have a basic understanding of what they are. For example:
1) Many combinatorics questions can be answered simply by thinking them through. Say, for the Olympic 100 meter dash finals, if 8 sprinters compete, how many different medal podium finishes (Gold-Silver-Bronze) are possible? Well, there are 8 runners who can win. When any one of them wins, there would be 7 runners left to finish second; and for every silver medalist, there would be six runners left to take bronze. So, the answer would be 8*7*6, or 336 potential finishes.
2) Others may be even more theoretical. I've seen a few Data Sufficiency questions that hinge on whether the number of objects to be arranged could be negative; say, How many ways can a chemist arrange N test tube holders on a rack? 1) N^2 = 16. Well, even though N could be 4 or -4, you can't arrange -4 items, as the number of test tubes can't be negative, so statement 1 would be sufficient.
If you can afford a couple hours to feel reasonably comfortable with items like the above, you'll give yourself a fighting chance on the harder problems, and be able to knock out any moderate-level combinatorics problems that you may face. Overall, it's probably a lower priority than a lot of things out there, but if you have a few hours it's probably worthwhile.
Brian Galvin
GMAT Instructor
Chief Academic Officer
Veritas Prep
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