user: Question: In the sixth round, what is the sum of fixtures when there were more than 15 clubs involved?
Columns: ['Round', 'Date(s)', 'Clubs from the previous round', 'Clubs involved', 'Fixtures']
Types: ['text', 'text', 'text', 'real', 'real']
Sample row: ['First round', '3 September 2006', 'none', 127.0, 60.0]
SQL:

assistant: SELECT SUM("Fixtures") FROM "Table" WHERE "Round" = 'sixth round' AND "Clubs involved" > 15.0;