user: Question: what's the average grid that has a time/retired piston and laps smaller than 15?
Columns: ['Driver', 'Constructor', 'Laps', 'Time/Retired', 'Grid']
Types: ['text', 'text', 'real', 'text', 'real']
Sample row: ['François Cevert', 'Tyrrell - Ford', 59.0, '1:43:51.991', 5.0]
SQL:

assistant: SELECT AVG("Grid") FROM "Table" WHERE "Time/Retired" = 'piston' AND "Laps" < 15.0;