What was the Sat 21 Aug time for the rider whose Tues 24 Aug time was 20' 38.40 109.680mph?
CREATE TABLE "2010_post_classic_senior_class_practice_" (
  "rank" real,
  "rider" text,
  "sat_21_aug" text,
  "mon_23_aug" text,
  "tues_24_aug" text,
  "wed_25_aug" text,
  "thurs_26_aug" text,
  "fri_27_aug" text,
  "sat_28_aug" text
);
SELECT "sat_21_aug" FROM "2010_post_classic_senior_class_practice_" WHERE "tues_24_aug"='20'' 38.40 109.680mph';