2012 AMC 10B Problem 15

Problem 15 of 25IntermediateAlgebra Counting & Probability

In a round-robin tournament with

6

teams, each team plays one game against each other team, and each game results in one team winning and one team losing. At the end of the tournament, the teams are ranked by the number of games won. What is the maximum number of teams that could be tied for the most wins at the end of the tournament?

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Solution

They would have to share

\binom 62 = 15

wins. This means we cannot have a

6

way tie as that would be

2.5

wins per team. If we had a

5

way tie, each team could have

3

wins, which is possible if one team loses all of its games, and out of the

5

winning teams, they each split their games. If we label the teams from

1

5,

and designate team

6

to lose all their games. To get a

5

way tie, we could have each team

1\le x \le 5

beat team

x+1 \mod 5

and team

x+2 \mod 5,

as well as team

6.

Thus, the correct answer is D .

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