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Resources for learning number properties

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Intern
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Resources for learning number properties [#permalink] New post 05 Mar 2015, 18:31
My GMAT is in 2 weeks and I have covered nearly all quant. However, I still struggle with number properties. I understand the basic even-odd/ positive negative problems, however I'm having real hard time with divisibility and primes. I learn through the MGMAT books and tbh, the 5th book on number properties in the MGMAT series largely went over my head (save for combinatorics/probability).

I even saw a tutor for the same but I guess it's just my bad luck that the tutor was very bad at explaining the concepts. She was great at solving questions but I couldn't really explain how to apply the concepts and approach the questions. I sat through a 3-hour number properties webinar by a prep company and most of it was just gibberish to me. Their approach was too formulaic and I just couldn't translate into real hard math that'd help me solve questions.

My question is, is there somewhere (on this forum or web, a guidebook, thread) something that'd help me tackle number properties as quickly and painlessly possible? Specifically, divisibility and primes? I believe my quant prep is complete save for this chink in my armor.

Thanks a bunch!
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Re: Resources for learning number properties [#permalink] New post 05 Mar 2015, 21:43
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Hi GhostA,

Chances are that you ALREADY know a bunch of Number Property rules (whether you realize it or not). I'm hoping that you can answer a few questions about the work that you've done so far:

1) What resources have you used (besides the MGMAT books)?
2) How well do you normally perform on DS questions? How did you do on DS during your last CAT?
3) Can you recognize Number Property vocabulary when it appears? Can you list any common NP words?

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Re: Resources for learning number properties [#permalink] New post 06 Mar 2015, 07:55
Two things I would say here:

One is that I think a big part of number property questions is just having good strategies on DS, since the questions appear more on DS and since they tend to lend themselves to picking numbers, etc. You often do not need to get too technical with the math and break everything down into primes on those questions. Often just good strategic number picking (with a healthy dose of DS know-how) is enough on these questions.

That said, I personally don't like ManhattanGMAT's prime box technique and prefer something that Kaplan used to teach (not sure if they still do). I am not sure if I can do this justice in a post, but let me try. Basically the concept is that a multiple divided by a factor must equal an integer. If you think about that, it is obvious, but it is more about how you apply it. So on a question that asks is 10! a multiple of 2^6, 10! is the multiple and 2^6 is the factor. We don't know yet if 10! is a multiple of 2^6 or if 2^6 is a factor of 10!, but if you break both down into primes it is fairly easy to see - basically you are trying to see if 10! divided by 2^6 would result in an integer. After breaking 10! down you will see that there are 8 2's that will be in the numerator of that equation and of course 2^6 is just 6 2's. So there are enough 2's in the numerator to cancel out the ones in the denominator and if there is no denominator left then the result will definitely be an integer and thus 10! is a multiple of 2^6 and 2^6 is a factor of 10!. Basically you put the multiple on the top, factor on the bottom, and then break everything down into primes to see if you have enough in the numerator to cancel what you have in the denominator.

I am not sure which Kaplan book you might find to give you a better explanation of that, but it is the technique that I use and the one that I teach to my students. Its not the only thing that you might use on divisibility and primes questions, but it would handle many of the tougher ones. But again to reiterate, often these questions, especially when they appear in DS, are really better approached by just good effective number picking.
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Re: Resources for learning number properties   [#permalink] 06 Mar 2015, 07:55
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