2016 AMC 12B Problem 20

Attempt Problem 20 of the 2016 AMC 12B below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2016 AMC 12B solutions, or check the answer key.

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20.

A set of teams held a round-robin tournament in which every team played every other team exactly once. Every team won 1010 games and lost 1010 games; there were no ties. How many sets of three teams {A,B,C}\{A,B,C\} were there in which AA beat B,B, BB beat C,C, and CC beat A?A?
385385
665665
945945
11401140
13301330
Answer: A
Concepts:graph theorycomplementary countingcombinations
Small hint:
Each team plays 2020 others, so there are 2121 teams and (213)=1330\binom{21}{3}=1330 triples in all
Big hint:
A non-cyclic triple has exactly one team that beats the other two; count those and subtract from the total
Solution:
Since each team won 1010 and lost 10,10, there are 2121 teams and (213)=1330\binom{21}{3}=1330 triples. A triple is not cyclic exactly when one team beats both others. Choosing that team (2121 ways) and 22 of the 1010 teams it beat gives 21⋅(102)=21⋅45=94521\cdot\binom{10}{2}=21\cdot45=945 non-cyclic triples. Thus the cyclic triples number 1330−945=385.1330-945=385.
Thus, the correct answer is A.
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