Below is an context that describes a sql query, paired with an question that provides further information. Write an answer that appropriately completes the request.  # noqa: E501
### Context:
CREATE TABLE table_name_3 (fleet_size INTEGER, type VARCHAR, introduced VARCHAR)
### Question:
What is the average fleet size for the Shunter type introduced in 1959?
### Answer:
SELECT AVG(fleet_size) FROM table_name_3 WHERE type = "shunter" AND introduced = "1959"