SCHEMA:
CREATE TABLE table_name_15 (play_offs INTEGER, total VARCHAR, league VARCHAR, player VARCHAR)
USER:
What is the mean number of play-offs when the league number was bigger than 18, where the player was John Grant and the total number was bigger than 25?

QUERY:
SELECT AVG(play_offs) FROM table_name_15 WHERE league > 18 AND player = "john grant" AND total > 25