Eight gymnastics players numbered 1 through 8 underwent a training camp where they were coached by three coaches - Xena, Yuki, and Zara. Each coach trained at least two players. Yuki trained only even numbered players, while Zara trained only odd numbered players. After the camp, the coaches evaluated the players and gave integer ratings to the respective players trained by them on a scale of 1 to 7, with 1 being the lowest rating and 7 the highest.
The following additional information is known.
1. Xena trained more players than Yuki.
2. Player-1 and Player-4 were trained by the same coach, while the coaches who trained Player-2, Player-3 and Player-5 were all different.
3. Player-5 and Player-7 were trained by the same coach and got the same rating. All other players got a unique rating.
4. The average of the ratings of all the players was 4.
5. Player-2 got the highest rating.
6. The average of the ratings of the players trained by Yuki was twice that of the players trained by Xena and two more than that of the players trained by Zara.
7. Player-4's rating was double of Player-8's and less than Player-5's. For how many players the ratings can be determined with certainty?
6
Step-by-Step Solution
Key idea: This is a constrained integer partition and group-assignment problem. We recognise it because players must be assigned to coaches under parity restrictions, then rated with integers subject to sum, average, and distinctness constraints.
Step 1: Determine group sizes and averages.
Let be the number of players trained by Xena, Yuki, and Zara respectively.
Total players = 8. Each coach trains at least 2 players ().
Clue 1 says .
The only integer partition of 8 into three parts where one part is strictly greater than another is .
Since , we must have . The remaining two are 2. So and .
Now for averages. Let be the average ratings.
Clue 6: and .
Clue 4: Overall average is 4, so Total Sum = .
Equation: .
Substitute: .
.
Therefore, and .
Group Sums: , , .
Step 2: Assign players to coaches.
Yuki (Y) trains only even numbers. Zara (Z) trains only odd numbers.
Clue 3: P5 and P7 are same coach, same rating. Both are odd, so they belong to Z.
Since and , we have .
Clue 2: P1 and P4 are same coach. P1 is odd, P4 is even. Only Xena can train both. So .
Clue 2 also says coaches for P2, P3, P5 are all different. P5 is Z.
Remaining coaches for P2, P3 are X and Y.
P2 is even, so P2 cannot be Z. P2 could be Y or X.
P3 is odd, so P3 cannot be Y. P3 could be X or Z.
But P5 is already Z. Since coaches for {P2, P3, P5} are distinct, P3 cannot be Z.
Therefore, P3 must be X.
This leaves P2 to be Y.
Current assignment: Z={5,7}, X={1,3,4}, Y={2}.
Y needs 2 players total. Remaining evens: {6, 8}. One goes to Y, one to X (to complete X's 4).
Clue 7: and .
Possible integers: If . If (invalid as ).
So and .
Where is P8? If P8 were in Y, then . Impossible (max 7).
So P8 must be in X. This forces P6 into Y.
Final Groups: Z={5,7}, Y={2,6}, X={1,3,4,8}.
Step 3: Calculate certain ratings.
Known: .
Y sum = 12. Clue 5 says P2 got highest rating. Max possible is 7. So .
Then .
X sum = 12. Current X sum = .
So .
Used ratings: {1, 2, 4, 5, 7}. Remaining available: {3, 6}.
Since , the set is definitely , but no clue distinguishes them.
Certain ratings: P2(7), P4(2), P5(4), P6(5), P7(4), P8(1).
Uncertain: P1, P3.
Count of certain ratings = 6.
Answer: 6