2012 AMC 8 Problem 14

Attempt Problem 14 of the 2012 AMC 8 below, then check your answer against the video solution and professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2012 AMC 8 solutions, or check the answer key.

All problems are used with official legal permission of the Mathematical Association of America (MAA).

14.

In the BIG N, a middle school football conference, each team plays every other team exactly once. If a total of 2121 conference games were played during the 20122012 season, how many teams were members of the BIG N conference?
6 6
7 7
8 8
9 9
10 10
Answer: B
Concepts:combinationstriangular number
Small hint:
List how many games are added when each new team joins.
Big hint:
With nn teams, the number of games is n(n−1)2\frac{n(n-1)}{2}.
Video solution:
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Written solution:
Each of the NN teams plays N−1N-1 games. However, 22 teams play each game, so multiplying NN and N−1N-1 would be twice the number of games. Therefore, we know N(N−1)=42.N(N-1) = 42. This leads to N2−N+0.25=42.25N^2 -N +0.25 = 42.25 which implies (N−0.5)2=6.52.(N-0.5)^2 = 6.5^2. In turn, this suggests: N−0.5=6.5N-0.5 = 6.5N=7 N = 7
Thus, the answer is B .
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