Find all School-related info fast with the new School-Specific MBA Forum

It is currently 23 Jul 2014, 02:38

Close

GMAT Club Daily Prep

Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track
Your Progress

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History

Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.

Events & Promotions

Events & Promotions in June
Open Detailed Calendar

distinguish perm/comb

  Question banks Downloads My Bookmarks Reviews Important topics  
Author Message
TAGS:
Manager
Manager
User avatar
Joined: 08 Sep 2010
Posts: 59
Followers: 0

Kudos [?]: 7 [0], given: 4

GMAT ToolKit User
distinguish perm/comb [#permalink] New post 08 Nov 2011, 14:14
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,
Manager
Manager
avatar
Status: SC SC SC SC SC.... Concentrating on SC alone.
Joined: 20 Dec 2010
Posts: 242
Location: India
Concentration: General Management
GMAT Date: 12-30-2011
Followers: 3

Kudos [?]: 42 [0], given: 47

GMAT Tests User
Re: distinguish perm/comb [#permalink] New post 08 Nov 2011, 19:33
Keep this rule and example in mind. You wont get confused.

Permutation - order matters - bigger value - so ABC, ACB, BAC, BCA, CAB, CBA all count in the final answer.

Combination - order doesnt matter - small value - so ABC = ACB = BCA = BAC = CAB = CBA = Considered as one value. Account as 1 value.

Second question is a permutation problem.

Bcos the order matters. Every different order count to a particular schedule.
So 6 * 5 * 4 * 3 * 2 is the answer for q2.

_________________

D- Day December 30 2011. Hoping for the happiest new year celebrations !

Aiming for 700+

Kudo me if the post is worth it

Manager
Manager
avatar
Status: SC SC SC SC SC.... Concentrating on SC alone.
Joined: 20 Dec 2010
Posts: 242
Location: India
Concentration: General Management
GMAT Date: 12-30-2011
Followers: 3

Kudos [?]: 42 [0], given: 47

GMAT Tests User
Re: distinguish perm/comb [#permalink] New post 08 Nov 2011, 19:47
And the first problem.

Totally we are asked to count the possible positions for set of 3 vaccancies.

1 in math dept, for which we have 7 choices.
2 positions in CSE dept. Identical positions. So one cannot occupy 2 positions and also the order doesn't matter. i.e AB = BA = 1 position. If there is a distinction between two positions then we can classify them as 2 different items. Since they are identical it is considered as 1 position. So it is a combination problem.

So for the 2 positions in CSE dept the possibilities are 10 * 9 / 2 = 45.
Taking them together 7*45 = 315.

Hope it helps

_________________

D- Day December 30 2011. Hoping for the happiest new year celebrations !

Aiming for 700+

Kudo me if the post is worth it

Expert Post
1 KUDOS received
Veritas Prep GMAT Instructor
User avatar
Joined: 16 Oct 2010
Posts: 4566
Location: Pune, India
Followers: 1029

Kudos [?]: 4454 [1] , given: 162

Re: distinguish perm/comb [#permalink] New post 09 Nov 2011, 03:46
1
This post received
KUDOS
Expert's post
386390 wrote:
Hi guys, having a little trouble distinguishing the two.

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

Combination is 'selection/picking people out of a group'
Permutation is 'arrangement'

Try using the terms selection and arrangement.


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?

We are given that the 2 seats are identical i.e. you just need to select 2 people. You don't have two different positions (e.g. a professor and an assistant professor). The positions are identical so there is no arrangement here. If the positions were not identical, then we would select two people (e.g. Mr A and Mr. B) and then arrange the two people in the two positions i.e. Mr A is professor and Mr. B is assistant professor OR Mr A is assistant professor and Mr B is professor. We would have 2 different arrangements in that case. That would have been a permutation. Right now, since the two positions are exactly the same, you only have to select two people. This is just a combination.


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!

Focus on the question - always. It asks you the number of different schedules she can make. Are the following 2 schedules different or not?
Mon - Mr A
Tue - Mr B
Wed - Mr C
Thu - Mr D
Fri - Mr E

and

Mon - Mr E
Tue - Mr A
Wed - Mr C
Thu - Mr D
Fri - Mr B

I hope you agree that the two schedules are different. So here, you choose 5 people out of 6 and then arrange the 5 on 5 different days. Let's say, I chose Mr A, B, C, D and E. Now I need to arrange them on Mon, Tue, Wed, Thu and Fri. Hence we need to choose and arrange here. This is a permutation.


Thanks,

_________________

Karishma
Veritas Prep | GMAT Instructor
My Blog

Save $100 on Veritas Prep GMAT Courses And Admissions Consulting
Enroll now. Pay later. Take advantage of Veritas Prep's flexible payment plan options.

Veritas Prep Reviews

Re: distinguish perm/comb   [#permalink] 09 Nov 2011, 03:46
    Similar topics Author Replies Last post
Similar
Topics:
Experts publish their posts in the topic Need help understanding what method to tackle Perm/Comb ques ghostie 3 10 Dec 2013, 01:09
2 Distinguished between Vs Distinguished from sacmanitin 3 27 Jan 2010, 20:25
Distinguish iamba 4 22 May 2007, 20:43
Experts publish their posts in the topic Distinguish Idiom leeye84 6 08 Apr 2007, 17:42
Need sample questions on Probability & Perm./Comb. Pranay_ju 2 17 Jun 2006, 21:49
Display posts from previous: Sort by

distinguish perm/comb

  Question banks Downloads My Bookmarks Reviews Important topics  


cron

GMAT Club MBA Forum Home| About| Privacy Policy| Terms and Conditions| GMAT Club Rules| Contact| Sitemap

Powered by phpBB © phpBB Group and phpBB SEO

Kindly note that the GMAT® test is a registered trademark of the Graduate Management Admission Council®, and this site has neither been reviewed nor endorsed by GMAC®.