how important is combinatorics?

[kn2130]

Hello,

My test is in two weeks. I am not comfortable at all with combinatorics and whenever I see combinatorics or probability questions on practice tests, I guess and move on. I have taken 9 practice exams MGMAT and GMATPrep and my scores, increasing over time, have been in the 640 - 710 range. (Mostly in the 680 - 710 range in the last several exams). I thought I would start studying it after I reviewed the rest of the material, but my test is two weeks away, and I'm not sure how wise it would be to start trying to learn this stuff now. My target score is 700. Do I NEED to learn combinatorics to achieve my target score? Or should I just focus on continuing to build on my current knowledge before the test?

Thanks!

[josh@knewton]

Hi there!

This is an interesting question. The answer depends on how much raw time you'll have to devote to the test in these final 2 weeks.

If you spend all of your two weeks of prep time learning new math (which should be done slowly, meticulously, and carefully), you might fall into bad test-taking habits. On test day you do NOT want to get cocky and "justify" all the neat combinatorics you learned by thinking deeply for 5 minutes about a n-choose-k problem. You might get it correct, but if it's not within that 2-minute/question time limit, you're hurting your score.

In the final two weeks, you should take at least one more CAT (or a practice exam) because it is extremely important to maintain good test-taking habits. You're going to want to review the answers and figure out what you did incorrectly for that CAT, as well, so budget in time for both the test and the review. The review might take a while. If you can take an additional CAT, do that as well.

8-9 hours of sleep per night in the final two weeks is essential as well (or whatever amount you normally get to feel fantastic). Do not rely on an early bedtime the night before to replenish a stressful two weeks' worth of late-night cramming.

So if you have any spare GMAT time between your CATs, your CAT reviews, and your sleep, you should definitely bone up on some probability and combinatorics. If you haven't already, check out www.knewton.com/gmat for the free trial. We have great GMAT-specific content on combinatorics.

Remember though-your practice scores seem to indicate that you're already primed for what you need, WITHOUT the combinatorics. Unless you can budget additional time for combinatorics while still maintaining good practice test-taking habits, leave the "new math" for after the test.

Knewton love,
Josh

[kn2130]

Thanks so much Josh. This was very helpful! I think I will try to just get the very basics -- so that at least I'm making educated guesses when these questions pop up. (Speaking of which, how often DO these questions pop up at the 700 level?)

[Brian@VeritasPrep]

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.

[smar83]

Be careful !!

I gave my GMAT last week and my VERY 1ST question on the GMAT was from Permutation/combination !! How odd is that ? and guess what ? I did not study it at all and ended up guessing my very 1st question on quant. Panic struck and it just downhill from there ( all this is immaterial )... but if I were you, I would defn look up atleast some basics to be prepared.