Answer:
8
Explanation:
The green team has 11 9-year-olds, so the red team has 2 * 11 = 22 10-year-olds.
Since each team has 15 players, the red team has 15 - 22 = -7 more 10-year-olds than the maximum number of players it can have.
Therefore, the red team must have 22 - 7 = 15 - 7 = 8 9-year-olds.