Table:
text: 2008 Portuguese motorcycle Grand Prix
250cc Classification
250cc Classification
Rider Manufacturer Laps Time_ Grid
Manuel Poggiali Gilera 20 +1:27.438 15
Imre Toth Aprilia 25 +1 lap 22
Doni Tata Pradita Yamaha 25 +1 lap 23
Roberto Locatelli Gilera 23 Retirement 19
Mattia Pasini Aprilia 20 Collision 3
Russell Gómez Aprilia 9 Retirement 24
Fabrizio Lai Gilera 4 Retirement 18
Alex Debon Aprilia 3 Accident 7
Ho Wan Chow Aprilia 1 Accident 25

Question: What are the average laps of the 250cc classification of the 2008 Portuguese Motorcycle Grand Prix when Grid is Grid is 15?