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combination or permutation

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combination or permutation

by Gurpinder » Fri Nov 04, 2011 11:44 am
I know that its ordered vs. unordered that decides which one it is; but is that the bottom line?
can someone elaborate on what it means by ordered vs. unordered?
[u]Look at this question[/u] wrote: The principal of a high school needs to schedule observations of 6 teachers. she plans to visit one teacher each day for a work week (M-F) so will only have time to see 5 of the teachers. How many different observation schedules can she create?
To me there is no order here as the principal can meet any of the 5 teachers any day! The order in which she meets those "5" selected teachers does not matter. Hence, shouldnt this be a combination problem?
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by shankar.ashwin » Fri Nov 04, 2011 12:42 pm
Well, the only difference I see is in a combination problem we typically pick up more than 2 things from a group (so that their order does not matter)

Here the difference is we select only one teacher for each day. So when only one is selected permutation and combination would be the same since there is no ordering..

For the 1st day 6 choices, she picks up one - 6C1 = 6
2nd day, 5 choices - 5C1 = 5
3rd day 4 choices, 1 to be selected = 4C1 = 4

So, this essentially translated to 6! (which is also 6P6).

If the problem were to select 5 teachers from 6 in a single event, then we would have 6C5, but here we have 5 separate events and hence the difference.
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by GmatMathPro » Fri Nov 04, 2011 1:07 pm
The difference all lies in the exact wording of the question. The word "schedule" implies that order matters. For example, if you have to choose 2 classes out of five to take from school, choosing math and literature is the same as choosing literature and math. The difference is trivial, because we're just talking about WHICH classes you take. However, taking math on monday and literature on friday is different from taking literature on monday and math on friday. Once we add this second dimension of time, order matters. How many ways can we choose two classes out of five: combinations. How many ways can we schedule two classes out of five: permutations. It's all about the context.

The same thing is going on here. How many groups of 5 out of 6 teachers the principal can meet with is a separate question from how many ways she can schedule them. Imagine you're at your job and you know you have 5 clients to meet with that day. You go into work and ask your secretary if there have been any changes to the schedule. She says, "nope". But then, to your surprise, your 5pm client shows up at 8am. You go out there and say "What the hell? I thought you said there weren't any changes to the schedule" and she says "There weren't! it's still the same five clients today!" Something tells me you'd be looking for a new secretary.
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by Gurpinder » Tue Nov 08, 2011 2:23 pm
Thanks Pete.

So the takeaway would be to pay close attention to the context and look for those keywords that hint at distinction between cases?
"Do not confuse motion and progress. A rocking horse keeps moving but does not make any progress."
- Alfred A. Montapert, Philosopher.
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