How long does the one that stars with son bou of middle difficulty, and on a stage number larger than 13 last?
CREATE TABLE "stages" (
  "stage" real,
  "start" text,
  "arrival" text,
  "duration" text,
  "difficulty" text
);
SELECT "duration" FROM "stages" WHERE "stage">13 AND "difficulty"='middle' AND "start"='son bou';