A small sports league has three divisions: Division A, Division B, and Division C. The average age of players in Division A is 25 years, in Division B is 20 years, and in Division C is 15 years. The number of players in Division A is half the number of players in Division B, and the number of players in Division B is three times the number of players in Division C. What is the average age of all players in the league?