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_12 (began_operation INTEGER, length__km_ VARCHAR, stations VARCHAR)
### Question:
What is the average operation beginning with a length of ampang and over 27 stations?
### Answer:
SELECT AVG(began_operation) FROM table_name_12 WHERE length__km_ = "ampang" AND stations > 27