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 01-25-2006, 04:17 PM #1 Member   Join Date: Nov 2005 Location: Swarthmore Posts: 659 nPr and nCr Could someone explain the difference for me and maybe give an example? nPr is where the probability must be done in a certain way and nCr is any way? i dont really understand this. could someone explain please =) Reply
 01-25-2006, 04:51 PM #2 Junior Member   Join Date: Dec 2005 Location: santo domingo, dominican republic Posts: 146 uh i'm not sure how you guys say it in english but nPr is for permutation (?) which is the probability when the order matters. nCr is combination, which is the probability when the order does not matter. Reply
 01-25-2006, 05:49 PM #3 Member   Join Date: Nov 2005 Location: Swarthmore Posts: 659 yeah but how do you determine when order matters. i dont really understand that cocept Reply
 01-25-2006, 06:11 PM #4 Senior Member   Join Date: Aug 2004 Location: New York City Posts: 3,962 How do you even remember that permutation = matter orders. I always get confused about which is which. Is there a device I can use to memorize that? Reply
 01-25-2006, 07:51 PM #5 Junior Member   Join Date: Dec 2005 Location: santo domingo, dominican republic Posts: 146 not really...the words in the problem say it. for example, if you have to find how many diff 'combinations' you can get of 3 out of 12 pizza toppings , then that's just combination because the order in which you put the toppings doesn't matter. however, if it's a problem like this: in how many ways can be chosen a president, vice president and secretary from a group of 30 students? in this case order does matter since it's not the same choosing a president and choosing a vice president. i hope you understood, i translated these examples from my math in spanish book. :S Reply
 01-25-2006, 08:52 PM #6 Junior Member   Join Date: Dec 2005 Posts: 165 nPr = n! / ((n-r)!) nCr = n! / ((n-r)!*r!) Reply
 01-25-2006, 10:50 PM #7 Member   Join Date: Nov 2005 Location: Oregon/ Duke Posts: 742 I'm pretty sure only one of them ever shows up on the SAT (from a crap load of practice test experience and two real tests +psat). I think it's nCr. I could be wrong. Reply
 01-27-2006, 04:40 PM #8 Senior Member   Join Date: Aug 2004 Location: college Posts: 1,258 I wouldn't be surprised if either showed up. Permutation is a more natural concept for our minds I think. Reply
 01-27-2006, 06:13 PM #9 Member   Join Date: Jul 2005 Location: Houston,Tx Posts: 472 Do you guys know any sites or specific pages in the blue book where I can practice these types of questions? Reply
 01-27-2006, 11:18 PM #10 Member   Join Date: Nov 2005 Posts: 393 Just use your TI83 #people/things nCr #of spots but if order matters then #people/things nPr #of spots Reply
 01-14-2011, 01:55 PM #11 New Member   Join Date: Jan 2011 Posts: 1 How to remember nCr and nPr nCr : Out of N, Choose R : Just Choose, When I choose I don't care about order. Or you can remember that permute means "change the order of". In addition remember that there will certainly be more ways in which to do things when order matters, hence when you permute, you lose one of the terms in the denominator, thus yielding a larger number of options when you permute. Reply
 01-14-2011, 07:49 PM #12 Junior Member   Join Date: Aug 2010 Posts: 73 I usually remember it like this: -Use nPr when order matters. For example, imagine stacking scoops of ice cream in an ice cream CONE. You can choose to put chocolate, vanilla, and strawberry in a certain order since the cone only stacks one way. However... -Use nCr when order does NOT matter. For example, imagine placing scoops of ice cream in a BOWL. Here, the order does NOT matter because you can place the ice cream in the bowl without stacking them on top. You can just get different combinations in the bowl without worrying about the order. Reply
 01-14-2011, 07:53 PM #13 Senior Member   Join Date: Jul 2010 Location: New York Posts: 1,912 Here is a practice question: I have 7 different colored marbles. I choose 4 from the bag. How many different possibilities are there? Reply
 01-14-2011, 07:56 PM #14 Senior Member   Join Date: Jul 2010 Location: New York Posts: 1,912 First marble can be any of the 7. Second marble can any of the 6 remaining marbles. Third " " " 5 Fourth " " " 4 Hence: 7*6*5*4 =30*28 = 840 But wait, I am overcounting! Divide by 4!=4*3*2=24 35 Reply
 01-14-2011, 09:12 PM #15 Junior Member   Join Date: Nov 2010 Posts: 203 Combinations are often used for choosing members of a committee that has no hierarchy, versus choosing people for president, vice president, secretary slots in a club. Reply

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