If a sample shows only one-fourth of its estimated original quantity of 14c, how old is it? think about half-lives; don't try to just plug into the decay equation. (you shouldn't need to plug into it at all for this problem.

The half-life of C-14 is approximately 5730 years. This means that the quantity contained in the sample becomes half of its original value after 5730 years. After another 5730 years, the quantity becomes 1/4 of its original value, and so on.

In our problem, the sample shows only one-fourth of its estimated original quantity, this means that  its age is twice 5730 years: