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2012 AMC 12B Problem 16

Problem 16 of 25IntermediateCounting & Probability

Amy, Beth, and Jo listen to four different songs and discuss which ones they like. No song is liked by all three. Furthermore, for each of the three pairs of the girls, there is at least one song liked by those two girls but disliked by the third. In how many different ways is this possible?

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Solution

Each song is liked by exactly one of the three pairs, by a single girl, or by no one. Every pair must be represented. Case 1:1: every song is liked by a pair. One pair gets two of the four songs ((42)=6\binom42=6 ways, and 33 choices for which pair), and the other two pairs get one song each (22 ways). This gives 362=36.3\cdot6\cdot2=36. Case 2:2: three songs go to the three pairs (one each) and the fourth song is liked by a single girl or no one. Assigning the four songs to these four roles gives 4!=244!=24 ways, and the leftover role has 44 options (Amy, Beth, Jo, or no one): 244=96.24\cdot4=96. The total is 36+96=132.36+96=132. Thus, the correct answer is B.

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Concepts: casework · multiplication principle

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