EXAMPLE 4.43

Downloading music from the Internet. The multiplication rule is just common sense made formal. For example, suppose that 30% of Internet users download music files, and 70% of downloaders say they don’t care if the music is copyrighted. So the percent of Internet users who download music (event A) and don’t care about copyright (event B) is 70% of the 30% who download, or

(0.7)(0.3) = 0.21 = 21%

The multiplication rule expresses this as

P(A and B) = P(A) × P(B | A)

= (0.3)(0.7) = 0.21