When 21' 18.87 106.209mph is Tuesday August 24th what is Monday August 23rd?
CREATE TABLE "2010_lightweight_manx_grand_prix_practic" (
  "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_29_aug" text
);
SELECT "mon_23_aug" FROM "2010_lightweight_manx_grand_prix_practic" WHERE "tues_24_aug"='21'' 18.87 106.209mph';