In the sixth round, what is the sum of fixtures when there were more than 15 clubs involved?
CREATE TABLE "format_and_schedule" (
  "round" text,
  "date_s" text,
  "clubs_from_the_previous_round" text,
  "clubs_involved" real,
  "fixtures" real
);
SELECT SUM("fixtures") FROM "format_and_schedule" WHERE "round"='sixth round' AND "clubs_involved">15;