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counting and probability junior

Problem

In the BIG N, a middle school football conference, each team plays every other team exactly once. If a total of 21 conference games were played during the 2012 season, how many teams were members of the BIG N conference?
(A)
(B)
(C)
(D)
Solution
This problem is very similar to a handshake problem. We use the formula to usually find the number of games played (or handshakes). Now we have to use the formula in reverse. So we have the equation . Solving, we find that the number of teams in the BIG N conference is .
Final answer
B