How can I master the combinatorics/permutations/probability for good?

Question

Hi everyone,

Combinatorics, Permutations and Probability have ALWAYS been my weakest area in math. I think I was first taught the concepts in 5th grade (I'm not from the US, it's mandatory for everyone here to take a certain level of math) and ever since that first time, these subjects have been confusing me to no end. Whenever I try to actually learn, I never quiet settle them and in a few days I forget everything I learned.

Well I hadn't studied the subjects for at least 4 year before I started studying for GMAT. I cannot tell you how dissappointed I was when I found out I was going to be tested on them as well. I studied the material first on Magoosh, which made sense at the time. And then I wanted to solve the manhattan problems, so I thought I should go through the explanations first, just in case. Totally different strategy they have there and now I feel completely lost again.

I have about %60 accuracy of all the questions I solved so far (about 100 questions ).

At this point it's almost pure luck whether I will pick the right concept to approach the problem. Has anyone ever been in a similar situation and able to solve it? What should I be doing?

Thanks in advance!

bb · Answer

The big question is always whether you need to. If you are scoring Q48 and higher then sure. But if not, there may be a greater ROI covering other areas such as arithmetic and Geometry.

Tmoich · Answer

Hi, I thought of that, but I do score above Q48 pretty consistenly. I would love to be able to say that I'm just not going to make any more effort and see what I do with my current level but unfortunately I am stuck thinking that I have to know them so well before I take the exam.

BrentGMATPrepNow · Answer

Don't despair.
As bb already pointed out, you can still get a decent quant score without knowing ANY counting techniques (just guess the 1 or 2 questions on test day and use the time for other questions).
Alternatively, it's useful to know that the Fundamental Counting Principle (FCP) can be used to solve the MAJORITY of counting questions on the GMAT. For more information about the FCP, watch our free video: https://www.gmatprepnow.com/module/gmat-counting/video/775
We also have a complete (and free) video module that covers everything you need to know about counting questions on the GMAT: https://www.gmatprepnow.com/module/gmat-counting

Cheers,
Brent

AjiteshArun · Answer

It is extremely unlikely that test prep companies will use different  concepts  to solve questions. This is because introducing new concepts that are generally not tested on the exam is not efficient. Or maybe I should be clearer: most test takers end up using the same set of concepts. Different approaches using the same set of concepts are still possible though. If that is the case, you might just need to go through the particular approach that you're not comfortable with. Alternatively, you might not be very comfortable with the basic concepts.

That said, I think I saw just 1-2 P&C questions on the exam. As has been pointed out, the additional time that you're thinking of spending on P&C might be better spent on other things.

Have you taken a GMATPrep test yet? What are those scores like?

chetan2u · Answer

Hi,
as  bb  too pointed out, you generally require to be concerned about PROB/PERMUTATION/COMB, if you are hitting above 48 and intend to touch 50-51..

Having said that, It is also important what effect does it have on you if you face something that you dread. Is it possible seeing such a Qs and thereafter guessing on it can have an afffect on few of later Qs..
So maybe, you should touch upon all the 600-700 level Qs, because they are simpler and can be mastered with some efforts..
Also when you say that you have found different approach by different institutes, the approach can be different but concepts must be same... ONE may be finding it directly and the OTHER the opposite and then subtracting by 1..

Few immediate tips I can give you..

1) First try to do only 600-700 level Qs, so that you build up confidence on these topics.

2) while you do so, take your time and understand the meaning... you can race against time later when you have got hold of it..

3) More often than not, you can eliminate few wrong choices easily and make intelligent guess thereafter..

Just before I read this thread, I saw a prob Qs 700 level.. 
the Q :- 8 world leaders standing together in a row for a photo. prob of mr O not standing with mr X.

WAY 1:-  let the two be one , so total 7.. ways this 7 can be arranged is 7!.
also two ways they can be arranged within themselves, so 2*7!..
total ways 8!.. ans=1-2*7!/8..

WAY 2:- but say i did not know at all about this.. 
the choices given were
A. 1/8
B. 1/4
C. 1/2
D. 3/4
E. 7/8
I know from Q that there are 8 people, and mr O can have any of the seven standing next to him... 
and Mr X is just one of these 7, so prob any of other 6 are there is 6/7.. 
so ans should be close to 6/7 only D and E are left, why it should be<6/7 requires a bit more exercise
when you know this A,B and C can be eliminated straightway and now prob of getting your answer correct is 1/2 against 1/5..

4) once you are comfortable with 600-700 level, you can graduate to 700 level depending on your aptitude

Ofcourse, there are various other ways one can improve, you have to look for the way that suits you..
Chetan

EMPOWERgmatRichC · Answer

Hi Tmoich,

If Permutations, Combinations and Probability are the only categories that you really have trouble with, then you're in remarkably good shape for someone who's only completed about 100 practice questions. Those 3 categories are some of the rarest on Test Day (and they can sometimes be rather complex questions), so they're really not worth your efforts just yet (and possibly not much effort at all).

1) How long have you been studying?
2) What resources are you using?
3) Have you taken any FULL-LENGTH CATs yet? If you have, then how did you score (including the Quant and Verbal Scaled Scores)?

4) What is your goal score?
5) When are you planning to take the GMAT?

GMAT assassins aren't born, they're made,
Rich

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How can I master the combinatorics/permutations/probability for good?

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