Answer
23,000 years.
Work Step by Step
After one half-life, we are down to one-half the original amount.
After two half-lives, we are down to one-quarter the original amount.
After three half-lives, we are down to one-eighth the original amount.
After four half-lives, we are down to one-sixteenth the original amount.
The intensity decreased by a factor of 16.7 (from 100 percent to 6 percent). This is a little over 4 half-life periods. So the wood is a little over (4)(5730 years) old, roughly 23,000 years.
