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Finding 'N' when total permutations or combinations given

This topic has 3 member replies
msd_2008 Senior | Next Rank: 100 Posts Default Avatar
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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

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    parallel_chase Legendary Member Default Avatar
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    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.

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    logitech Legendary Member
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    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!

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    niraj_a Legendary Member Default Avatar
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    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.

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