Carbon-14 has a half-life of approximately 5,730 years. How many half-lives have occurred in a sample that contains less than 10 percent of the original amount of the parent isotope?

I don't really know my Biology anymore. But I'll take a guess at this answer.

Carbon-14 starts at 100% (no decay). Every half-life it goes down by half.

So one half-life = 50%

Two half-life = 25%

3 half-life = 12.5%

4 half-life = 6.25% (This one has less than 10 percent of the original parent isotope)

Thus the answer seems to be 4 half-lives.