question: How many episodes had rating/share (18-49) of 0.7/2 and a rating of 2.1?
answer: SELECT COUNT(episode_number) FROM table_25751274_2 WHERE rating / SHARE(18 - 49) = 0.7 / 2 AND rating = "2.1"
context: CREATE TABLE table_25751274_2 (episode_number VARCHAR, rating VARCHAR)