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?
Answer choices
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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 every song is liked by a pair. One pair gets two of the four songs ( ways, and choices for which pair), and the other two pairs get one song each ( ways). This gives
Case 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 ways, and the leftover role has options (Amy, Beth, Jo, or no one):
The total is
Thus, the correct answer is B.