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permutation

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permutation

by iwill » Mon Aug 31, 2009 9:12 am
Guys,
Please discuss this problem. Also please mention when we need to consider clockwise and anti clockwise arrangements as separate, in circular permutation.

In how many ways can 21 differently colored beads be stung on a necklace ?

1. (21)!
2. (20)!
3.(21)!/2
4.(20)!/2
5.19!

OA: 4
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by praky_rules » Mon Aug 31, 2009 4:07 pm
This problem is similar to the arrangement problem on a circular table except that you can view the necklace from either side. So each arrangement of the necklace constitutes two relative postions, one seen from front and other from the back. The number of possible arrangements = (n-1)!/2
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