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

It is currently 21 Oct 2014, 08:58

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

Combination ---- Help Needed

  Question banks Downloads My Bookmarks Reviews Important topics  
Author Message
TAGS:
Intern
Intern
avatar
Joined: 09 Sep 2004
Posts: 16
Followers: 0

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

Combination ---- Help Needed [#permalink] New post 15 Sep 2004, 14:00
Guys,

Just searching the archives and found this combination Problem.

In a group of 8 semifinalists, all but 2 will advance to the final round. If in the final round only the top 3 will be awarded medals then how many groups of medals winners are possible?

Read approach of Paul and ywilfred which cleared my concepts to some extent but still have some confusion. Let me explain the confusion.

This is the combination problem as the word group has been used which means combination. Now the answer is 8C6 * 6C3 = 560. Right.
Why didn't we assume that 2 particular persons say X and Y couldn't make upto the finals, and then calculate the combinations of 3 among 6.


Now few days ago same type of problem was discussed which was :

1. A woman has 11 close friends. Find the number of ways she can invite 5 of them to dinner :

a) Where 2 of the friends are married and will not attend separately.

Here in this problem we fixed the positions of the couple first.. i.e assumed the couples are A and B and then found the remaining 3 people from 9 people.
So, we have 9C3*2C2 + 9C5 = 210 ways

My question is that why not like the semi-finalists problem we take all the combinations of couples b/c we don't know who is married with who. i.e instead of taking 2C2 for couples , why not taking all the combinations of couples formed with 12 people.

Also this forum is using XCX notation for Combinations as well as Permutations , right?

Can anybody clear this confusion?
Director
Director
User avatar
Joined: 16 Jun 2004
Posts: 893
Followers: 1

Kudos [?]: 10 [0], given: 0

 [#permalink] New post 16 Sep 2004, 00:07
1. There could be some additional learning in the thread below which I thought will be a relevance and value
http://www.gmatclub.com/phpbb/viewtopic.php?t=4524

2. Permutations is described as nPr or n!/(n-r)! and combinations is expressed as nCr or n!/r!(n-r)!. Note in both cases r<=n.

3. The way to read the problem under discussion is that => from 8 it gets to 6 and from 6 it gets to 3. We will take the same flow for easy logic, hence it is 6 out of 8 (which is 8C6) and 3 out of 6 (which is 6C3) => totalling to 8C6*6C3 ways.

You would notice that complementary logic is best used in those situations when that is more convenient without loosing the crux of the logic.

In the couples proble, when 5 people are to selected and 2 of them are married. We dont need to really know which are these two. Why? because we can read the question simply as 2 out of 11 are couples and 9 others are not couples. If we need to choose 4 of these 11 people, I now know that I can select any 3 out of 9 and the couples OR any 5 out of the 9 and not the couples. Here the couples is the restraint added to the question and we take that into careful consideration. However, again in what order we select the 5 people also does not matter.
Intern
Intern
avatar
Joined: 09 Sep 2004
Posts: 16
Followers: 0

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

 [#permalink] New post 16 Sep 2004, 19:46
Thanks Venksune. I got it.

But in these type of problems , one needs to think a lot. I don't know how come these problems can be solved in less than 2 minutes. Probably after practice it can be possible.
  [#permalink] 16 Sep 2004, 19:46
    Similar topics Author Replies Last post
Similar
Topics:
3 Experts publish their posts in the topic Need help on Quants:Combination nishadparkhi 3 14 Mar 2013, 00:35
Experts publish their posts in the topic Easy Combination, but need help! johnnybear 8 26 Apr 2011, 12:50
Experts publish their posts in the topic probability math(combinations (nCk)) help needed succybuzz 5 18 Jan 2010, 15:39
1 Combination Problem HELP dhushan 2 07 Sep 2009, 19:33
Please help: More permutation/combination/probability need saumster 2 11 Aug 2006, 12:55
Display posts from previous: Sort by

Combination ---- Help Needed

  Question banks Downloads My Bookmarks Reviews Important topics  


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®.