When did mika heinonen susanna dahlberg win mixed doubles?
CREATE TABLE table_name_55 (
    year INTEGER,
    mixed_doubles VARCHAR
)
SELECT AVG(year) FROM table_name_55 WHERE mixed_doubles = "mika heinonen susanna dahlberg"
Below are sql tables schemas paired with instruction that describes a task. Using valid SQLite, write a response that appropriately completes the request for the provided tables. ### Instruction: When did mika heinonen susanna dahlberg win mixed doubles? ### Input: CREATE TABLE table_name_55 (
    year INTEGER,
    mixed_doubles VARCHAR
) ### Response: SELECT AVG(year) FROM table_name_55 WHERE mixed_doubles = "mika heinonen susanna dahlberg"