Combinatorics Summary

Question

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.
https://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...

KarishmaB · Answer

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.

sdas · Answer

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

KarishmaB · Answer

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: https://www.gmatclub.com/forum/veritas-prep-resource-links-no-longer-available-399979.html#/categor ... er-wisdom/

Bunuel · Answer

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.

Yekrut · Answer

Thank you for posting the combinatorics questions Bunuel, it's something I need to improve

01dynamic · Answer

Thanks for the post..... one of Q areas i need to improve on......

bumpbot · Answer

Automated notice from GMAT Club BumpBot: 

A member just gave Kudos to this thread, showing it’s still useful. I’ve bumped it to the top so more people can benefit. Feel free to add your own questions or solutions.

 This post was generated automatically.

Combinatorics Summary

Prep Toolkit

Top 5 GMAT Debrief Videos

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia

My Rewards