posted by Rock on .
An anthropologist finds there is so little remaining Carbon-14 in a prehistoric bone that instruments cannot measure it. This means that there is less than 0.5% of the amount of Carbon-14 the bones would have contained when the person was alive. How long ago did the person die? Round your answer to the nearest thousand. (22,000, etc)
Find the half-life of C14. Call it n years
2^(-t/n) < .005
-t/n < ln(.005)/ln(2) = -7.64
t > 7.64n
check: 2^-7.64 = .005013, so after 7.64 half-lives, we are down to .5% of the original.