user: Question: What is the average operation beginning with a length of ampang and over 27 stations?
Columns: ['Line', 'Began operation', 'Stations', 'Length (km)', 'Terminals']
Types: ['text', 'real', 'real', 'text', 'text']
Sample row: ['1 September 1998', 24.0, 29.0, 'Kelana Jaya', 'Gombak']
SQL:

assistant: SELECT AVG("Began operation") FROM "Table" WHERE "Length (km)" = 'ampang' AND "Stations" > 27.0;