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## distinguish comb/perm

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Gurpinder GMAT Destroyer!
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distinguish comb/perm Tue Nov 08, 2011 3:08 pm
Elapsed Time: 00:00
• Lap #[LAPCOUNT] ([LAPTIME])
Hi guys, having a little trouble distinguishing the two.

Comb = unordered where order doesen't matter.
perm = order matters.

But, look at this questions:
Question wrote:
A certain university will select 1 of 7 candidates eligible to fill a position in the mathematics department and 2 of 10 candidates eligible to fill 2 identical positions in the computer science department. If none of the candidates is eligible for a position in both departments, how many different sets of 3 candidates are there to fill the 3 positions?
To me, the second part of the question looks like a perm. Since there are 2 seats and therefore 2 candidates (hence they are distinguishable) wouldn't it be 10!/8!=90?

I know I am wrong. Can someone clarify how I can make a better decision distinguishing the two.

Look at another one:
Quote:
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?

Am I missing some key rule here? or am I just losing it!

Thanks,

_________________
“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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shankar.ashwin GMAT Destroyer!
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Tue Nov 08, 2011 10:16 pm
Read the question carefully, the second part states that the 2 students fill up identical positions.(there is not differentiation between both of them). Its similar to "In how many ways can you pick 2 people from a group of 10". Here obviously the order you pick them up does not matter.

Since, given its an identical position, this would be a combination problem.

7C1 * 10C2 = 315 IMO

Gurpinder wrote:
Question wrote:
A certain university will select 1 of 7 candidates eligible to fill a position in the mathematics department and 2 of 10 candidates eligible to fill 2 identical positions in the computer science department. If none of the candidates is eligible for a position in both departments, how many different sets of 3 candidates are there to fill the 3 positions?
I think Pete clarified your other question..

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First, the content structure of the passage is as follows: It starts with a brief introduction, then presents a user's post about a confusion between combinations and permutations in two different scenarios, and ends with an expert's response to the user's query.
single_question_with_answer
the passage contains a single mathematical question about distinguishing between combinations and permutations, and an answer to that question is provided by an expert.
Given a text passage, determine:
    1. If it contains a mathematical question (including word problems or calculations)
    2. If an answer to that mathematical question is provided in the text

First, describe the content structure you found in 1-2 sentences.

Then provide your analysis in the following format:
TYPE: <one of the following values>
- NO_QUESTION
- SINGLE_QUESTION
- SINGLE_QUESTION_WITH_ANSWER
- MULTIPLE_QUESTIONS
- MULTIPLE_QUESTIONS_WITH_ANSWERS
system
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## distinguish comb/perm

This topic has 1 member reply
Gurpinder GMAT Destroyer!
Joined
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Posted:
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distinguish comb/perm Tue Nov 08, 2011 3:08 pm
Elapsed Time: 00:00
• Lap #[LAPCOUNT] ([LAPTIME])
Hi guys, having a little trouble distinguishing the two.

Comb = unordered where order doesen't matter.
perm = order matters.

But, look at this questions:
Question wrote:
A certain university will select 1 of 7 candidates eligible to fill a position in the mathematics department and 2 of 10 candidates eligible to fill 2 identical positions in the computer science department. If none of the candidates is eligible for a position in both departments, how many different sets of 3 candidates are there to fill the 3 positions?
To me, the second part of the question looks like a perm. Since there are 2 seats and therefore 2 candidates (hence they are distinguishable) wouldn't it be 10!/8!=90?

I know I am wrong. Can someone clarify how I can make a better decision distinguishing the two.

Look at another one:
Quote:
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?

Am I missing some key rule here? or am I just losing it!

Thanks,

_________________
“Do not confuse motion and progress. A rocking horse keeps moving but does not make any progress.”
- Alfred A. Montapert, Philosopher.

Need free GMAT or MBA advice from an expert? Register for Beat The GMAT now and post your question in these forums!
shankar.ashwin GMAT Destroyer!
Joined
02 Jan 2010
Posted:
966 messages
Followed by:
20 members
Thanked:
228 times
Tue Nov 08, 2011 10:16 pm
Read the question carefully, the second part states that the 2 students fill up identical positions.(there is not differentiation between both of them). Its similar to "In how many ways can you pick 2 people from a group of 10". Here obviously the order you pick them up does not matter.

Since, given its an identical position, this would be a combination problem.

7C1 * 10C2 = 315 IMO

Gurpinder wrote:
Question wrote:
A certain university will select 1 of 7 candidates eligible to fill a position in the mathematics department and 2 of 10 candidates eligible to fill 2 identical positions in the computer science department. If none of the candidates is eligible for a position in both departments, how many different sets of 3 candidates are there to fill the 3 positions?
I think Pete clarified your other question..

### Best Conversation Starters

1 varun289 31 topics
2 sana.noor 23 topics
3 killerdrummer 21 topics
4 Rudy414 19 topics
5 sanaa.rizwan 14 topics
See More Top Beat The GMAT Members...

### Most Active Experts

1 Brent@GMATPrepNow

GMAT Prep Now Teacher

199 posts
2 GMATGuruNY

The Princeton Review Teacher

134 posts
3 Jim@StratusPrep

Stratus Prep

106 posts
4 Anju@Gurome

Gurome

47 posts
user
First, the content structure of the passage is as follows: It starts with a brief introduction, then presents a user's post about a confusion between combinations and permutations in two different scenarios, and ends with an expert's response to the user's query.

TYPE: SINGLE_QUESTION_WITH_ANSWER
The passage contains a single mathematical question about distinguishing between combinations and permutations, and an answer to that question is provided by an expert.