What is the Opponent name at memorial stadium • minneapolis, mn on 10/20/1928?
CREATE TABLE "schedule" (
  "date" text,
  "opponentnum" text,
  "site" text,
  "result" text,
  "attendance" text
);
SELECT "opponentnum" FROM "schedule" WHERE "site"='memorial stadium • minneapolis, mn' AND "date"='10/20/1928';