Those Permutations and Combinations

[shane.]

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So the current example I had trouble with on a practice exam was, "how many ways can a team of 3 people be selected from a group of 5 people."


I got this problem wrong the first time around, but now I think I get it. I was having a hard time distinguishing whether or not to use a permutation or combination.

Let's see if I get this correct. Since the team is made up of three indistinguishable positions the order of the team does not matter. That is, a team of ABC is the same as a team of CAB. So this means we want to use a combination. Yes?

What technique do you use to differentiate when to use a permutation and when to use a combination?

 

[z3u2]

What technique do you use to differentiate when to use a permutation and when to use a combination?

you should be able to differentiate that from reading the question
it is usually pretty clear

 

[Streetwolf]

Forget about the proper names. Just think of them as 'selection with order' and 'selection without order'. When you throw in names like combination and permutation it just confuses you haha.

Since you want a TEAM of 3 people it doesn't matter how you select them.

If the problem said that the first person chosen would be the leader, the second would be the 2nd in command, and the third would be the 3rd in command, then order WOULD matter and you would use a permutation (selection with order).

 

[alalha]

Forget about the proper names. Just think of them as 'selection with order' and 'selection without order'. When you throw in names like combination and permutation it just confuses you haha.

Since you want a TEAM of 3 people it doesn't matter how you select them.

If the problem said that the first person chosen would be the leader, the second would be the 2nd in command, and the third would be the 3rd in command, then order WOULD matter and you would use a permutation (selection with order).

perfect explanation.......

 

[predent0305]

Hi guys, I am really having trouble with permutations. I am using the destroyer right and there seem to be 3 variations of the same equation- I just don't get when to use which one. Please HELP

1) n!/ r!(n-r)!

2) n!/ (n-r)!

I am not sure why he multiplies by r! in the first one but not the second one.

 

[Streetwolf]

Normally I'd say do a search but it was on the first page.

http://forums.studentdoctor.net/showthread.php?t=424802

 

[predent0305]

Thanks so much 🙂

 

[Dentalicous1234]

Normally I'd say do a search but it was on the first page.

http://forums.studentdoctor.net/showthread.php?t=424802

thank you streetwolf! this is good stuff 🙂