Permutations or Combinations?Date: 01/09/2003 at 14:44:35 From: Aniela Subject: Finite math When given a permutation or combination problem, is there anything SPECIFIC to look for to know whether it is one or the other? For example, are there any "key words" to look for? This also relates to probability. How does one know when to use permutations or combinations? Date: 01/10/2003 at 11:17:05 From: Doctor Ian Subject: Re: Finite math Hi Aniela, This is a good question, but I don't think it has the answer you were hoping for. English is a pretty slippery language, so as a rule, trying to use little formulas ("When I see the word _____, they must be talking about _____ instead of _____") is a bad idea. The only reliable way to know whether a problem is asking for permutations or combinations is to write down a few examples of what's being asked for (e.g., how many ways can you make double-dip cones from a specified number of ice-cream flavors), and then try to establish whether or not order makes a difference. Sometimes that isn't even something that you can tell by looking at the words in the problem. Often you just have to use common sense. For example, if you're asked to make up committees from a collection of prospective members, it obviously doesn't matter how you order the members. But there aren't any special words that will tip you off to that fact. Note that sometimes you need to find permutations OF combinations, or combinations OF permutations: Fast Food Combinations - Dr. Math FAQ http://mathforum.org/dr.math/faq/faq.mcdonalds.html This is another reason that trying to use keywords to decide what to do isn't such a great idea. I hope this helps. Write back if you'd like to talk more about this, or anything else. - Doctor Ian, The Math Forum http://mathforum.org/dr.math/ |
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