user: Question: Which MotoGP winner has a 250cc winner of Marco Simoncelli, and a Round of 15?
Columns: ['Round', 'Date', 'Grand Prix', 'Circuit', '125cc winner', '250cc winner', 'MotoGP winner', 'Report']
Types: ['real', 'text', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: [1.0, '12–13 April', 'Qatar Grand Prix ‡', 'Losail', 'Andrea Iannone', 'Héctor Barberá', 'Casey Stoner', 'Report']
SQL:

assistant: SELECT "MotoGP winner" FROM "Table" WHERE "250cc winner" = 'Marco Simoncelli' AND "Round" = 15.0;