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Topic: Pulling Names Out Of A Hat (Read 1192 times) |
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ThudnBlunder
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Pulling Names Out Of A Hat
« on: Jan 21st, 2009, 5:57pm » |
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Each year n couples meet for a Christmas party and to exchange gifts. Each of the 2n people write their name on a piece of paper and put it into a hat. They then take turns pulling names out of the hat and give their gift to the person whose name they drew. But if anyone chooses either their own name or the name of their partner, everyone puts the name they drew back into the hat and draws again.
a) How many different permissible ways of choosing the 2n names are there?
b) What is the expected number of attempts required?
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| « Last Edit: Jan 21st, 2009, 9:17pm by ThudnBlunder » |
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THE MEEK SHALL INHERIT THE EARTH.....................................................................er, if that's all right with the rest of you.
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balakrishnan
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Re: Pulling Names Out Of A Hat
« Reply #1 on: Jan 30th, 2009, 10:52am » |
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This is similar to this
I won't spoil the fun by giving the answer. The expected number is just the inverse of the probability.
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| « Last Edit: Jan 30th, 2009, 12:38pm by balakrishnan » |
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