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briana.img
 one year ago
Find the number of ways to listen to 4 different CDs from a selection of 15 CDs.
briana.img
 one year ago
Find the number of ways to listen to 4 different CDs from a selection of 15 CDs.

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briana.img
 one year ago
Best ResponseYou've already chosen the best response.0I thought it was 15*14*13*12 but it's not

jim_thompson5910
 one year ago
Best ResponseYou've already chosen the best response.1That would be the answer if order mattered, but it does not matter

jim_thompson5910
 one year ago
Best ResponseYou've already chosen the best response.1Let's say you had 4 CDs labeled A,B,C,D there are 4! = 4*3*2*1 = 24 ways to arrange the four letters (eg: ABCD or ACBD) you have to divide the answer you got by 24 to account for the fact that order doesn't matter

briana.img
 one year ago
Best ResponseYou've already chosen the best response.0aaaah okay okay so 1365

jim_thompson5910
 one year ago
Best ResponseYou've already chosen the best response.1Smaller example Let's say you had 4 CDs (A,B,C,D) and you pick 2 of them. order doesn't matter 4*3 = 12 if order mattered. The full list of choices is AB, BA AC, CA AD, DA BC, CB BD, DB CD, DC notice how the list above is double of what it should be. We've counted everything twice. Which is why we divide by 2! = 2*1 = 2 to get 12/2 = 6. There are 6 ways to pick the two CDs where order doesn't matter

jim_thompson5910
 one year ago
Best ResponseYou've already chosen the best response.1yes it is 1365 an alternative route is to use the combination formula \[\Large C(n,r) = \frac{n!}{r!*(nr)!}\]
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