Permutations - Norman Twins Problem

dtowler

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Nov 5, 2010
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The question is:
Ten students have been nominated for the positions of secretary, treasurer, social convenor, and fundraising chair. In how many ways can these positions be filled if the Norman twins are running and plan to switch positions on occasion for fun since no one can tell them apart?
The answer is supposed to be 6144. <-- How do you obtain that number?


There are essentially 3 scenarios:
1: if 0 Norman twins are elected, in which case = 8P4
2: if 1 Norman twin is elected, in which case = 9P4 - 8P4 (I think)
3: if both Norman twins are elected. This is the part I can't quite figure out. I thought it would be 10P4/2!

The sum of these 3 numbers is my answer. (Clearly wrong though)

Is this correct? It doesn't match the answer in the back of my textbook.
 
dtowler said:
The question is:
Ten students have been nominated for the positions of secretary, treasurer, social convenor, and fundraising chair. In how many ways can these positions be filled if the Norman twins are running and plan to switch positions on occasion for fun since no one can tell them apart?
The answer is supposed to be 6144. <-- How do you obtain that number?


There are essentially 3 scenarios:
1: if 0 Norman twins are elected, in which case = 8P4
2: if 1 Norman twin is elected, in which case = 9P4 - 8P4 (I think)
3: if both Norman twins are elected. This is the part I can't quite figure out. I thought it would be 10P4/2!

The sum of these 3 numbers is my answer. (Clearly wrong though)

Is this correct? It doesn't match the answer in the back of my textbook.

For #3 - start with the assumption that there are two positions to fill (other two hasbeen taken up by the twins) with 8 students. (That is not the complete situation though - just a start)
 
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