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If each of the 12 teams participating in a certain tournament plays exactly one game with each of the other teams, how many games will be played?


     A. 144 

     B. 132  

     C. 66 

     D. 33  

     E. 23


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答案:
C

Arithmetic E

lementary Combinatorics

Since each of the 12 teams will play exactly one game with each of the other teams, the number of games that will be played is equal to the number of selections of 2 teams, without regard to order, from 12 teams. This is the number of combinations of 12 teams taken 2 at a time, which is equal to image.png.

Alternatively, each of the 12 teams will play each of the 11 other teams. The product (12)(11) counts each of the games twice since, for example, this product separately counts “Team A plays Team B” and “Team B plays Team A”. Therefore, the number of games that will be played isimage.png .

The correct answer is C.


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