Answer
$1.0\times10^4$ years.
Work Step by Step
Following the example, let x be the number of elapsed half-lives. The problem tells us that $0.29=(\frac{1}{2})^x$.
$$ln 0.29=x ln (1/2)$$
$$x=1.786$$
The elapsed time is 1.786 multiplied by the half-life of carbon-14, which is 5730 years.
About $1.0\times10^4$ years have elapsed.
