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Permutations - Order Matters

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Permutations - Order Matters [#permalink] New post 14 Jun 2009, 11:17
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I was struggling with when to use the permutation formula and when to use the combination formula in various problems. The definitions of permutations and combinations are are sometimes not clear to beginners. The easiest way that I have found to use the permutation formula is when the question states that order matters. Looking for key words in the sentence like:
*Rank
*Arrange
*Place
*Chose
*Pick
and many others signify that order matters. If order matters then use the permutation formula.

Examples:

*In how many ways can 8 CD’s be arranged on a shelf?

*If a softball league has 10 teams, how many different end of the season rankings are possible? (Assume no ties).

*How many different arrangements can be made using two of the letters of the word TEXAS if no letter is to be used more than once?

* A teacher has 15 students and 5 are to be chosen to give demonstrations. How many different ways can the teacher choose the demonstrators given the following conditions.

*In how many ways can a sorority of 20 members select a president, vice president and treasury, assuming that the same person cannot hold more than one office.

All of these questions are answered using the permutation formula: \(nPr=\frac{n!}{(n-r)!}\)

This is something that has helped me, hopefully it helps others.
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Re: Permutations - Order Matters [#permalink] New post 14 Jun 2009, 15:06
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Thanks!
I added your post to Walker's Sticky: combination-and-probability-references-56486.html
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Re: Permutations - Order Matters [#permalink] New post 25 Oct 2009, 18:34
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I3igDmsu wrote:
* A teacher has 15 students and 5 are to be chosen to give demonstrations. How many different ways can the teacher choose the demonstrators given the following conditions.


I think that in this example order doesn't matter. Could somebody prove/reject, pls

Thanks in advance.
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Re: Permutations - Order Matters [#permalink] New post 03 Jan 2010, 09:06
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My 2 cents is that this forum is a fantastic way to find resources and hone skills and an awful way to try to gander basic concepts from others' posts. I blundered around not really understanding Permutations and such until reading this link (which certainly came from someone in here who I would give credit to if I could)

http://www.themathpage.com/aPreCalc/per ... ions-2.htm
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Re: Permutations - Order Matters [#permalink] New post 01 Dec 2013, 16:47
Hello from the GMAT Club BumpBot!

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Re: Permutations - Order Matters [#permalink] New post 07 Dec 2013, 05:21
Vyacheslav wrote:
I3igDmsu wrote:
* A teacher has 15 students and 5 are to be chosen to give demonstrations. How many different ways can the teacher choose the demonstrators given the following conditions.


I think that in this example order doesn't matter. Could somebody prove/reject, pls

Thanks in advance.



yes order has no work... only combination is req..
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Re: Permutations - Order Matters [#permalink] New post 05 Jan 2015, 11:16
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Permutations - Order Matters [#permalink] New post 05 Jan 2015, 21:50
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WasimAkramKhan wrote:
Vyacheslav wrote:
I3igDmsu wrote:
* A teacher has 15 students and 5 are to be chosen to give demonstrations. How many different ways can the teacher choose the demonstrators given the following conditions.


I think that in this example order doesn't matter. Could somebody prove/reject, pls

Thanks in advance.



yes order has no work... only combination is req..


I3igDmsu has done a great job of giving sample questions where Order Matters :)

He is right that being aware of certain keywords helps you determine whether Order Matters in a question or not. However, please note that the words 'Pick' and 'Choose' indicate that order DOES NOT matter. So, the list of keywords provided by I3igDmsu needs to be corrected in this regard.

This also probably explains why WasimAkramKhan asked the follow-up question, which had the keyword 'choose'. In this question, order DOES NOT matter. It is therefore a selection (combination) question.


- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

While the keywords present in the question statement are of help in clearing the doubt about whether Order Matters or not, nothing beats conceptual understanding of WHY order matters in a particular case and not in another.

Let us try to answer this 'WHY' in the question quoted by WasimAkramKhan above.

Let the 15 students in question be named A, B, C . . . O

The question here asks, in how many ways can the teacher choose the 5 demonstrators?

So, what can the possible groups of demonstrators be?

Some examples are: (ABCDE), (ABCDF) . . . (BCDEO) etc.

Essentially, we are choosing 5 students out of 15 students here and giving them the label of 'demonstrators'. So, this is a SELECTION question. Order doesn't matter here. The answer will be 15C5

This question also illustrates what we said above: The word 'CHOOSE' indicates that the given question involves Selection (Combination) and that Order doesn't matter.

Let us now consider a different question:

A teacher has 15 students and 5 are to be chosen to give demonstrations. In how many ways can the demonstrations be made?

Do you see the difference between this question and the previous one? ('ways to choose the demonstrators' v/s 'ways to make the demonstrations')

In the second question, we will have two tasks to perform:
1. Select a group of 5 students to demonstrate
2. Within a selected group of students, decide the order in which the students make the demonstrations.

For example, let the chosen group of demonstrators be: A, B, C, D and E

Now, within this group, we will have to make another decision: Who will demonstrate first of all? Who will be the second to demonstrate and so on . . .

Basically, ORDER MATTERS in this case. So, this is a Permutation question and the answer to this question will be: 15P5

The Takeaways from this discussion:
When in doubt about whether the given question is about Permutation or about Combination,
1. Look for certain helpful keywords
2. Try to imagine the scenario given in the question (maybe with the help of an example) and ask yourself: DOES ORDER MATTER here?


Hope this helps! :-D

Japinder
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Permutations - Order Matters   [#permalink] 05 Jan 2015, 21:50
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