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Combination Permutation Strategy

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myfish Rising GMAT Star Default Avatar
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Combination Permutation Strategy Post Fri Apr 27, 2012 12:21 pm
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  • Lap #[LAPCOUNT] ([LAPTIME])
    I understand that the Combination and Permutation-formula can be used interchangeably as they are essentially the same. However, I stumble repeatedly over one type of question where I am not sure which question I should be using.

    Example 1:
    If there are 2 empty chairs left in an auditorium but there are 8 people who need to be seated, in how many number of different ways can those 8 people be seated on 2 empty chairs?

    Example 2:
    How many different ways are there to choose 2 candidates from 8 possible candidates?

    Both questions deal with 8 into 2. However, one is done via Permutation and one is Combination! WHY? How can i identify which one to use, without guessing?

    Thanks a lot to all the geniuses out there that can answer this sufficiently.

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    Lasve Rising GMAT Star
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    Post Fri Apr 27, 2012 2:58 pm
    I use a simple Trick:
    PERMUTATION = Position count, that means that if we have to "permutate" threee people (A;B;C) it is differetn wheter we have ABC or BCA so ABC ≠ BCA


    COMBINATION = Position does NOT count, that means that if you have to "combine" three people (A;B;C)
    Wether you have ABC or BCA is actually THE SAME!

    Combination is a sort of IN or OUT Question
    Permutation not only wants to know WHO seated, but also in What order!

    Hope I helped to clarify your problem a little!

    Thanked by: myfish
    Lasve Rising GMAT Star
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    Post Fri Apr 27, 2012 2:58 pm
    I use a simple Trick:
    PERMUTATION = Position count, that means that if we have to "permutate" threee people (A;B;C) it is differetn wheter we have ABC or BCA so ABC ≠ BCA


    COMBINATION = Position does NOT count, that means that if you have to "combine" three people (A;B;C)
    Wether you have ABC or BCA is actually THE SAME!

    Combination is a sort of IN or OUT Question
    Permutation not only wants to know WHO seated, but also in What order!

    Hope I helped to clarify your problem a little!

    Shalabh's Quants Really wants to Beat The GMAT!
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    Post Sat Apr 28, 2012 6:46 am
    myfish wrote:
    I understand that the Combination and Permutation-formula can be used interchangeably as they are essentially the same. However, I stumble repeatedly over one type of question where I am not sure which question I should be using.

    Example 1:
    If there are 2 empty chairs left in an auditorium but there are 8 people who need to be seated, in how many number of different ways can those 8 people be seated on 2 empty chairs?

    Example 2:
    How many different ways are there to choose 2 candidates from 8 possible candidates?

    Both questions deal with 8 into 2. However, one is done via Permutation and one is Combination! WHY? How can i identify which one to use, without guessing?

    Thanks a lot to all the geniuses out there that can answer this sufficiently.
    Thumb Rule-- Whenever Qn. uses words like Arrangement, Distinct, Unique. One must calculate Permutation & when Qn. uses words like Selection, Choose, calculate combination.

    _________________
    Shalabh Jain,
    e-GMAT Instructor

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