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# Combinatorics Summary

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Intern
Joined: 13 Jan 2012
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Combinatorics Summary [#permalink]

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29 Jan 2012, 23:28
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Am not trashing MGMAT's guides...in fact, I'm a fan. But, I think the combinatorics section could've been written a bit more clearer, coz I remember being able to solve these problems using easier techniques in the past.

I googled, and chanced upon this resource...that made my day.
http://www.mathsisfun.com/combinatorics ... tions.html

Clear, Crisp & Concise formulas for the 4 categories of problems:

[n is the size of the pool, r is the # of objects we're selecting from the pool]

1) ORDER MATTERS = Y, REPETITIONS = allowed
Formula: n raised to the power r

2) ORDER MATTERS = Y, REPETITIONS = not allowed (nPr)
Formula: n! / (n-r)!

3) ORDER MATTERS = N, REPETITIONS = allowed
Formula: (n+r-1)!/( (r!) (n-1)!)

4) ORDER MATTERS = N, REPETITIONS = not allowed (nCr)
Formula: n! / ( (r!) (n-r)! )

Thought I'd share this for others struggling to remember what you learned years ago...

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Veritas Prep GMAT Instructor
Joined: 16 Oct 2010
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Re: Combinatorics Summary [#permalink]

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30 Jan 2012, 02:32
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fxsunny wrote:
Am not trashing MGMAT's guides...in fact, I'm a fan. But, I think the combinatorics section could've been written a bit more clearer, coz I remember being able to solve these problems using easier techniques in the past.

I googled, and chanced upon this resource...that made my day.
http://www.mathsisfun.com/combinatorics ... tions.html

Clear, Crisp & Concise formulas for the 4 categories of problems:

[n is the size of the pool, r is the # of objects we're selecting from the pool]

1) ORDER MATTERS = Y, REPETITIONS = allowed
Formula: n raised to the power r

2) ORDER MATTERS = Y, REPETITIONS = not allowed (nPr)
Formula: n! / (n-r)!

3) ORDER MATTERS = N, REPETITIONS = allowed
Formula: (n+r-1)!/( (r!) (n-1)!)

4) ORDER MATTERS = N, REPETITIONS = not allowed (nCr)
Formula: n! / ( (r!) (n-r)! )

Thought I'd share this for others struggling to remember what you learned years ago...

If you are hoping to solve all P&C questions using formulas, you will be disappointed. There are a ton of them. One word in the question changes and your formula becomes useless (though there is a new formula for it then). You need to understand some basic concepts of the topic. That will help you handle most of the questions.
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Re: Combinatorics Summary [#permalink]

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30 Jan 2012, 02:41
Only the 3rd combination I am finding difficult to explore. Rest was nicely explained in the link, thanks.Karishma, you mention about basics, apart from MGMAT guides and forum discussions, would you have any references for those? Pls advice
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Re: Combinatorics Summary [#permalink]

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30 Jan 2012, 02:58
sdas wrote:
Only the 3rd combination I am finding difficult to explore. Rest was nicely explained in the link, thanks.Karishma, you mention about basics, apart from MGMAT guides and forum discussions, would you have any references for those? Pls advice

Veritas has a book dedicated to Combinatorics and Probability. It is one of the most comprehensive sources for GMAT Combinatorics theory and practice questions.

Also, for the past few weeks, I have been discussing this topic extensively on my blog: http://www.veritasprep.com/blog/categor ... er-wisdom/
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Re: Combinatorics Summary [#permalink]

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30 Jan 2012, 03:38
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sdas wrote:
Only the 3rd combination I am finding difficult to explore. Rest was nicely explained in the link, thanks.Karishma, you mention about basics, apart from MGMAT guides and forum discussions, would you have any references for those? Pls advice

Try Combinatorics chapter of Math Book to have an idea about the staff that is tested on the GMAT: math-combinatorics-87345.html

Also try some questions on combinations to practice:
DS: search.php?search_id=tag&tag_id=31
PS: search.php?search_id=tag&tag_id=52
Hard questions on combinations and probability with detailed solutions: hardest-area-questions-probability-and-combinations-101361.html

In addition, note that usually the GMAT combination/probability questions are fairly straightforward and as practice shows you won't encounter more than 2-3 questions from this field.

Hope it helps.
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Re: Combinatorics Summary [#permalink]

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14 Mar 2012, 16:33
Thank you for posting the combinatorics questions Bunuel, it's something I need to improve

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Concentration: Finance, Strategy
Schools: Chicago (Booth) - Class of 2014
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Re: Combinatorics Summary [#permalink]

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21 Mar 2012, 11:35
Thanks for the post..... one of Q areas i need to improve on......
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Re: Combinatorics Summary [#permalink]

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Re: Combinatorics Summary   [#permalink] 18 Aug 2014, 01:04
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# Combinatorics Summary

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