help with probability

This topic has 1 member reply
allenkt Rising GMAT Star Default Avatar
14 Apr 2007
36 messages
help with probability Post Fri Apr 20, 2007 6:03 pm
Elapsed Time: 00:00
    For some reason I really seem to struggle with probability questions. Here is one that was on my GMATPrep that I just can't figure out. It seems like its relatively but something just isn't clicking. Here is the question (as best I remember it but the numbers are correct):

    There are 10 students in a class and they are going to pair off in groups of 2. How many different combinations are possible? The answer is 45 but I don't know how they got there. I think it would have something to do with factorials but I don't know.

    Need free GMAT or MBA advice from an expert? Register for Beat The GMAT now and post your question in these forums!
    allenkt Rising GMAT Star Default Avatar
    14 Apr 2007
    36 messages
    Post Fri Apr 20, 2007 6:10 pm
    I probably should just delete my last post but in case someone else is struggling with this I wanted to give the solution. As soon as I wrote out the problem and realized it would involve factorials I vaguely remembered a formula I had seen. Once I found it and plugged it in it provided the solution.

    The formula is C(n,r) = n! / r! (n-r)!

    So solving for C(10,2) = 10! / 2! (10-2)! which simplifies down to 9 x 5 / 1 and that equals 45.

    Hopefully I'll remember this during the test....

    Best Conversation Starters

    1 hazelnut01 61 topics
    2 NandishSS 21 topics
    3 rsarashi 16 topics
    4 GMATinsight 15 topics
    5 Hmna 14 topics
    See More Top Beat The GMAT Members...

    Most Active Experts

    1 image description GMATGuruNY

    The Princeton Review Teacher

    114 posts
    2 image description Rich.C@EMPOWERgma...


    99 posts
    3 image description Matt@VeritasPrep

    Veritas Prep

    89 posts
    4 image description Brent@GMATPrepNow

    GMAT Prep Now Teacher

    87 posts
    5 image description DavidG@VeritasPrep

    Veritas Prep

    68 posts
    See More Top Beat The GMAT Experts