Maths Olympiad Prep

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Combinatorics Difficulty 6.9 National olympiad Find the answer

At a chess tournament the winner gets 1 point and the defeated one 0 points. A tie makes both obtaining 12\frac{1}{2} points. 14 players, none of them equally aged, participated in a competition where everybody played against all the other players. After the competition a ranking was carried out. Of the two players with the same number of points the younger received the better ranking. After the competition Jan realizes that the best three players together got as many points as the last 9 players obtained points together. And Joerg noted that the number of ties was maximal. Determine the number of ties.

A number or a short expression. Spacing and $ signs are ignored.

Solution

1. Define the Problem and Variables:
- There are 14 players, each playing against every other player.
- Points: Win = 1, Loss = 0, Tie = 12\frac{1}{2}.
- The best three players (set AA) have the same total points as the last nine players (set CC).
- The number of ties is maximal.

2. Total Number of Games:
- Each player plays against 13 others.
- Total number of games:
(142)=14×132=91 \binom{14}{2} = \frac{14 \times 13}{2} = 91

3. Points Distribution:
- Total points in the tournament:
91 games×1 point per game=91 points 91 \text{ games} \times 1 \text{ point per game} = 91 \text{ points}
- Let f(X)f(X) be the total points of players in set XX.
- Given: f(A)=f(C)f(A) = f(C).

4. **Points Calculation for Sets AA and CC:**
- Points for set AA:
f(A)3+3×11=36 f(A) \leq 3 + 3 \times 11 = 36
(3 games among themselves and 11 games each against others)
- Points for set CC:
f(C)36 f(C) \geq 36
(36 games among themselves)

5. Equality Condition:
- Since f(A)=f(C)f(A) = f(C), we have:
f(A)=36andf(C)=36 f(A) = 36 \quad \text{and} \quad f(C) = 36
- Players in AA win all games against BCB \cup C.
- Players in BB win all games against CC.

6. Games within Sets:
- **Set AA:**
- Cannot have all three players draw (would result in same score).
- Possible scenario: v1v3v_1 \to v_3, v1v2v_1 \leftrightarrow v_2, v2v3v_2 \leftrightarrow v_3.
- Result: v1>v2>v3v_1 > v_2 > v_3 with 2 draws.

- **Set CC:**
- Net games won by nine players must sum to zero.
- At least 4 distinct wiw_i with the same sign.
- Minimum sum of positive wiw_i: 1+2+3+4=101 + 2 + 3 + 4 = 10.
- Possible scenario: v9{v1,v2,v3,v4}v_9 \to \{v_1, v_2, v_3, v_4\}, v8{v1,v2,v3}v_8 \to \{v_1, v_2, v_3\}, v7{v1,v2}v_7 \to \{v_1, v_2\}, v6v1v_6 \to v_1.
- Result: 10 games won, 26 draws.

7. **Set BB:
- Unique pair with the same score.
- Additional draw: 1 draw.

8. Total Number of Draws:**
- Draws in set AA: 2
- Draws in set CC: 26
- Draws in set BB: 1
- Total draws:
2+26+1=29 2 + 26 + 1 = 29

The final answer is 29\boxed{29}

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.