When 20 is the rr4 points what is the lowest rr3 points?
CREATE TABLE "round_robin" (
  "team_name" text,
  "races" real,
  "won" real,
  "rr1_pts" real,
  "rr2_pts" real,
  "rr3_pts" real,
  "rr4_pts" text,
  "total_pts" real,
  "ranking" real
);
SELECT MIN("rr3_pts") FROM "round_robin" WHERE "rr4_pts"='20';