Welcome! Check out our free B-School Guides to learn how you compare with other applicants.
Login or Register

Finding 'N' when total permutations or combinations given

This topic has 3 member replies
msd_2008 Rising GMAT Star Default Avatar
Joined
29 Jun 2008
Posted:
66 messages
Thanked:
1 times
Finding 'N' when total permutations or combinations given Post Fri Dec 05, 2008 3:47 am
Elapsed Time: 00:00
  • Lap #[LAPCOUNT] ([LAPTIME])
    Guys,

    Can anyone please explain to me how to find 'n' when total permutations or combinations have been given?
    For eg:- 10 C 4 = 210. What if the problem said N C 4 = 210, find value of N?
    Similarly, 10 P 4 = 5040. What if the question said N P 4 = 5040, find the value of N ?


    Regards
    MSD

    _________________
    When the going gets tough, the tough gets going.

    Need free GMAT or MBA advice from an expert? Register for Beat The GMAT now and post your question in these forums!
    parallel_chase GMAT Titan Default Avatar
    Joined
    20 Jun 2007
    Posted:
    1153 messages
    Followed by:
    1 members
    Thanked:
    140 times
    Target GMAT Score:
    V50
    Post Fri Dec 05, 2008 11:54 am
    Now those are pretty big values to calculate the combinations

    Try with easy one, GMAT only tests logic , therefore you wont be required to calculate such values


    Here is the basic concept:

    5C2 = 10

    NC2 = 10

    N! / (N-2)! 2! = 10

    N! / (N-2)! = 10* 2!

    N! = N*(N-1)*(N-2)!

    N*(N-1)*(N-2)! / (N-2)! = 20

    N*(N-1) = 20

    N^2 -N -20 = 0

    N^2 -5N + 4N -20 = 0

    (N-5) (N+4) = 0

    either N can be 5 or -4, since we are calculating for combinations there can be no negative values, hence the answer is 5.

    You can apply the same method in the original question.

    NC4 = 210

    N!/(N-4)!*4! = 210

    N!/(N-4)! = 210 * 2!

    N! = N*(N-1)*(N-2)*(N-3)*(N-4)!

    N*(N-1)*(N-2)*(N-3)*(N-4)! / (N-4)! = 5040

    N*(N-1)*(N-2)*(N-3) = 5040

    You can solve for N to find its value.

    If GMAT gives you such big numbers, which is very unlikely but nevertheless you can insert the answer options to find the solution.

    Hope this helps.

    _________________
    No rest for the Wicked....

    logitech GMAT Titan
    Joined
    20 Oct 2008
    Posted:
    2136 messages
    Followed by:
    20 members
    Thanked:
    232 times
    GMAT Score:
    730
    Post Fri Dec 05, 2008 12:07 pm
    parallel_chase wrote:
    GMAT only tests logic , therefore you wont be required to calculate such values
    Priceless intel!

    _________________
    LGTCH
    ---------------------
    "DON'T LET ANYONE STEAL YOUR DREAM!"

    niraj_a GMAT Destroyer! Default Avatar
    Joined
    01 Jun 2008
    Posted:
    708 messages
    Followed by:
    1 members
    Thanked:
    13 times
    Post Fri Dec 05, 2008 12:19 pm
    true. if they need you to calc values then just work backwards from the answer choices, it will be much faster.

    Best Conversation Starters

    1 abhasjha 56 topics
    2 tanvis1120 23 topics
    3 qwerty12321 18 topics
    4 aditya8062 18 topics
    5 Nijo 18 topics
    See More Top Beat The GMAT Members...

    Most Active Experts

    1 image description GMATGuruNY

    The Princeton Review Teacher

    189 posts
    2 image description Brent@GMATPrepNow

    GMAT Prep Now Teacher

    139 posts
    3 image description Jim@StratusPrep

    Stratus Prep

    60 posts
    4 image description David@GMATPrepNow

    GMAT Prep Now Teacher

    56 posts
    5 image description Jon@Admissionado

    Admissionado

    46 posts
    See More Top Beat The GMAT Experts