What is the position of the player who is from the United States and has 30 minutes? 
CREATE TABLE "playoff_statistics" (
  "name" text,
  "nationality" text,
  "position" text,
  "appearances" real,
  "starts" real,
  "minutes" real,
  "goals" real
);
SELECT "position" FROM "playoff_statistics" WHERE "nationality"='United States' AND "minutes"=30;