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January 29th, 2017, 07:44 AM  #1 
Senior Member Joined: Jan 2016 From: Blackpool Posts: 104 Thanks: 2  Help on Combinatorics
Hey guys I am struggling to understand what the difference is between ordered and unordered selection between a set of elements. Could anyone please give me an example which i might find helpful to understand them? Thanks again and sorry for the spam! 
January 29th, 2017, 08:42 AM  #2  
Senior Member Joined: Sep 2015 From: USA Posts: 2,452 Thanks: 1337  Quote:
The set will be ordered if I care about which color was selected first, which was selected second, etc. In this case, $\{B, B, G\}$ is considered a distinct event from $\{G, B, B\}$ On the other hand if we only care about what balls we end up with after all the selections are made we have an unordered selection. In this case the two color triples above are considered the same event. Last edited by romsek; January 29th, 2017 at 09:36 AM.  
January 29th, 2017, 05:07 PM  #3  
Senior Member Joined: Feb 2010 Posts: 706 Thanks: 141  Quote:
1. Suppose we want to know how many committees of size 2 can be formed. There are 3. They are AB, AC, and BC. Notice that if the committee is Al and Bob, then Bob and Al is the same committee. 2. Suppose we want to know how many president, vicepresident pairs there are. There are 6. They are AB, BA, AC, CA, BC, CB. Notice that president Al and vicepresident Bob is different than president Bob and vicepresident Al. The first problem is called a combination. The second is called a permutation.  

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