What is the correct formula to use when posed a question such as: quot; A biologist wishes to estimate the mean number of teeth in an adult tiger...

What is the correct formula to use when posed a question such as: "A biologist wishes to estimate the mean number of teeth in an adult tiger shark. He wishes to generate a 99% confidence interval with a margin of error of 4 teeth. The standard deviation, σσ, is known to be 21.3. How many tiger sharks must he sample?"