A cereal company runs a certain advertisement in three media: newspaper, radio, and TV. Of the customers surveyed, 40 saw the advertisement on TV, 40 heard it on the radio, 30 read it in the newspaper, 20 saw it both in the newspaper and on TV, 20 know it both from TV and radio, 20 know it both from newspaper and radio, and 10 know it from all three media. How many customers were surveyed?