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March 14th, 2011, 11:11 PM  #1  
Newbie Joined: Aug 2009 Posts: 29 Thanks: 0  How can I understand permutation and probability question?
Especially on probability question. I found it difficult to understand a question that asked for its probability whether using permutation or combination. Example of final exam question: Quote:
 
March 14th, 2011, 11:27 PM  #2 
Senior Member Joined: Jul 2010 From: St. Augustine, FL., U.S.A.'s oldest city Posts: 12,211 Thanks: 521 Math Focus: Calculus/ODEs  Re: How can I understand permutation and probability questio
For i) we have 4 choices for the first digit, 3 for the 2nd 2 for the 3rd and 1 for the 4th: For ii) we have 4 choices for all 4 digits: 
March 15th, 2011, 01:56 AM  #3 
Senior Member Joined: Feb 2009 From: Adelaide, Australia Posts: 1,519 Thanks: 3  Re: How can I understand permutation and probability questio
Oops! You don't have four choices for the first digit; it can only be 1, 2, or 3. So you have (i) 18, and (ii) 192. The question you have to ask yourself is, "How are the valid possibilities constructed?" Can you construct them by permuting a list? Then think about permutations. Can you construct them by choosing from a list? Then think about combinations. Is it possible to select one element at a time, without any selection having an impact on later selections? Then think about a multiplication. If one selection does affect later selections, consider drawing a game tree of possibilities. 
March 15th, 2011, 05:49 AM  #4 
Senior Member Joined: Jul 2010 From: St. Augustine, FL., U.S.A.'s oldest city Posts: 12,211 Thanks: 521 Math Focus: Calculus/ODEs  Re: How can I understand permutation and probability questio
D'oh! 

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