• EMPOWERgmat Slider
    1 Hour Free
    BEAT THE GMAT EXCLUSIVE

    Available with Beat the GMAT members only code

    MORE DETAILS
    EMPOWERgmat Slider
  • Economist Test Prep
    Free Trial & Practice Exam
    BEAT THE GMAT EXCLUSIVE

    Available with Beat the GMAT members only code

    MORE DETAILS
    Economist Test Prep
  • e-gmat Exclusive Offer
    Free Resources for GMAT Preparation
    Practice Questions, Videos & webinars

    Available with Beat the GMAT members only code

    MORE DETAILS
    e-gmat Exclusive Offer
  • Target Test Prep
    5-Day Free Trial
    5-day free, full-access trial TTP Quant

    Available with Beat the GMAT members only code

    MORE DETAILS
    Target Test Prep
  • Kaplan Test Prep
    Free Practice Test & Review
    How would you score if you took the GMAT

    Available with Beat the GMAT members only code

    MORE DETAILS
    Kaplan Test Prep
  • Magoosh
    Magoosh
    Study with Magoosh GMAT prep

    Available with Beat the GMAT members only code

    MORE DETAILS
    Magoosh
  • PrepScholar GMAT
    5 Day FREE Trial
    Study Smarter, Not Harder

    Available with Beat the GMAT members only code

    MORE DETAILS
    PrepScholar GMAT
  • Veritas Prep
    Free Veritas GMAT Class
    Experience Lesson 1 Live Free

    Available with Beat the GMAT members only code

    MORE DETAILS
    Veritas Prep

Probabilities question

This topic has 1 expert reply and 0 member replies
allenkt Senior | Next Rank: 100 Posts Default Avatar
Joined
14 Apr 2007
Posted:
36 messages

Probabilities question

Post Sun Apr 29, 2007 7:08 pm
Elapsed Time: 00:00
  • Lap #[LAPCOUNT] ([LAPTIME])
    Admittedly, probability is a weak area for me because I don't remember ever taking anything like it in high school or college (never took statistics). I have a question about a formula. I recently learned the formula to be as follows:

    If you have n items, taken r at a time, the total number of possible combinations can be found as such:

    C(n,r) = n! / r! (n-r)!

    However in most of the problems I've seen nobody is using that exact formula. Instead they are using something else. I can't remember it exactly and I can't seem to find an example but it's something like

    n!/r! * n!

    What is difference in the two ways of finding the answer? Hopefully someone knows what I'm talking about because I'll admit I'm even starting to confuse myself...lol.

    Need free GMAT or MBA advice from an expert? Register for Beat The GMAT now and post your question in these forums!

    GMAT/MBA Expert

    Stacey Koprince GMAT Instructor
    Joined
    27 Dec 2006
    Posted:
    2228 messages
    Followed by:
    678 members
    Thanked:
    639 times
    GMAT Score:
    780
    Post Mon Apr 30, 2007 1:28 pm
    The formula you list below is for combination / permutation problems, which sometimes overlap with probability, but they aren't the same thing. I don't recognize your second formula - but the first is the classic combination formula (though there are a lot of others for both comb. and perm.).

    There are also a lot of methods to choose from in this area - and they're too involved for me to type out right here, unfortunately. If you are taking a course, ask your teacher about comb/perm in particular and how best to tackle these. If you are studying on your own, find combinations / permutations in whatever books you are using and use that method. (And if you have more than one book, pick the method that is most comfortable for you.)

    These don't show up a ton on the test, so don't worry about them too much.

    _________________
    Please note: I do not use the Private Messaging system! I will not see any PMs that you send to me!!

    Stacey Koprince
    GMAT Instructor
    Director of Online Community
    Manhattan GMAT

    Contributor to Beat The GMAT!

    Learn more about me

    Free Manhattan Prep online events - The first class of every online Manhattan Prep course is free. Classes start every week.

    Best Conversation Starters

    1 Vincen 139 topics
    2 lionsshare 51 topics
    3 Roland2rule 38 topics
    4 lheiannie07 35 topics
    5 AbeNeedsAnswers 35 topics
    See More Top Beat The GMAT Members...

    Most Active Experts

    1 image description Rich.C@EMPOWERgma...

    EMPOWERgmat

    119 posts
    2 image description Brent@GMATPrepNow

    GMAT Prep Now Teacher

    118 posts
    3 image description GMATGuruNY

    The Princeton Review Teacher

    105 posts
    4 image description Jay@ManhattanReview

    Manhattan Review

    103 posts
    5 image description DavidG@VeritasPrep

    Veritas Prep

    101 posts
    See More Top Beat The GMAT Experts