user: Question: What is the mean number of laps when the grid is less than 19 and time/retired is +21.3 secs?
Columns: ['Driver', 'Team', 'Laps', 'Time/Retired', 'Grid']
Types: ['text', 'text', 'real', 'text', 'real']
Sample row: ['Dario Franchitti', 'Team Green', 65.0, '1:58:40.726', 1.0]
SQL:

assistant: SELECT AVG("Laps") FROM "Table" WHERE "Grid" < 19.0 AND "Time/Retired" = '+21.3 secs';