When considering the lifetime of a radioactive species, a general rule of thumb is that after 10

Question:

When considering the lifetime of a radioactive species, a general rule of thumb is that after 10 half-lives have passed, the amount of radioactive material left in the sample is negligible. The disposal of some radioactive materials is based on this rule.
a. What percentage of the original material is left after 10 half-lives?
b. When would it be a bad idea to apply this rule?