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March 3rd, 2009, 11:38 AM   #1
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Combination permutation problem HELP!

a) You have 12 different post cards that you want to send five friends, each of whom can be sent any number of postcards. How many different choices do you have?



b) How many permutation of 1,2,3,4,5,6,7,8,9 have the property that 1 preceedes 2 preceedes 3 ?
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March 3rd, 2009, 12:55 PM   #2
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Re: Combination permutation problem HELP!

Q1.

Send the first postcard. How many choices do you have?
Send the second postcard. How many choices do you have?
And so on. Multiply.

Q2.

There are six equally likely orderings for 1, 2, 3. So one sixth of all permutations will have them in the order 1, 2, 3.
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March 5th, 2009, 06:03 AM   #3
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Quote:
Originally Posted by erogol
a) You have 12 different post cards that you want to send five friends, each of whom can be sent any number of postcards. How many different choices do you have?
If I understand this correctly, you can send anywhere between five and twelve cards, and send anywhere between one and eight cards to any one person. I think you'll have to proceed by (many) cases.

If you send eight to one person, then you'll have only one each for the other four. In how many ways can you choose the first eight, in how many ways can you choose the one person, and in how many ways can you distribute the other four?

Then move on to seven for one person, two for another, and so forth.

Then you'll have to account for choosing to send only eleven cards, etc, etc.

Good luck!
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