Internet sites often vanish or move so that references to them can’t be followed. In fact, 87% of Internet sites referred to in major scientific journals still work within two years of publication. Suppose we randomly select 7 Internet references from scientific journals.

a. Find the probability that all 7 references still work two years later.

b. What’s the probability that at least 1 of them doesn’t work two years later?

c. Explain why the calculation in part (a) may not be valid if we choose 7 Internet references from one issue of the same journal.