Question

An artifact contains 1/4 as much carbon-14 as the atmosphere. How old is the artifact

Answer 1

The artifact appears to have gone through 2 half-lives. 
One half-life for Carbon-14 is 5730 years.
After 5730 years, there would be 1/2 as much carbon-14
After another 5730 years, there would be 1/4 as much carbon-14

The answer is roughly 11,460 years