SELECT "U.S. viewers (millions)" FROM table_29531 WHERE "Written by" = 'Jack Sanderson'
How many millions of US viewers watched the episode written by Jack Sanderson?
CREATE TABLE table_29531 (
    "No. in series" real,
    "Title" text,
    "Directed by" text,
    "Written by" text,
    "U.S. viewers (millions)" text,
    "Original U.S. air date" text,
    "Prod. code" real
)