Below is an instruction that describes a task. Write a response that appropriately completes the request.

### Instruction:
What is the mean number of laps when the grid is less than 19 and time/retired is +21.3 secs?

### Inputs:
table_name: table, columns: ['Driver', 'Team', 'Laps', 'Time/Retired', 'Grid']

### Response:
SELECT AVG Laps FROM table WHERE Grid < 19 AND Time/Retired = +21.3 secs