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Possible Duplicate:
Number of different necklaces using $m$ red and $n$ white pebbles

I don't understand high level maths. Please try to do simple$\ldots$ I tried to hunt down the pattern but couldn't complete as I have my exam.

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    Please clarify whether you mean "necklace" in the technical sense of considering arrangements related by a rotation as equivalent, as mentioned in sdcvvc's comment under Jen-Ya's answer, or in the everyday sense where a necklace has a distinguished spot where it opens and closes, so that rotations do matter.2012-10-14

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[Edit] Sorry, I was not aware that necklaces are considered equivalent up to rotation. Thanks to sdcvvc for correcting

This solution is only right for situations, where rotations are not equivalent:

(n+m)! / (n! * m!)

You have n+m Beads and (n+m)! ways to order them. But you have to ignore the order of black beads and the order of white beads. There are n! permutations (possibilities to order) of black beads and m! permutations of white beads. So you have (n+m)! / (n! * m!) ways to order the beads.

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    Sorry, I was not aware of that! Thanks for correcting.2012-10-14