Asample of wood contains 12.5% of carbon-14.what is the estimated age of this sample?

How old is an artifact that contains 12.5 percent of its original carbon-14?
:
The generally accepted value for the half-life of Carbon-14 is 5730 yr
:
Let t = time for 12.5% of the artifact to remain
Let the initial amt = 1

:
1*2^(-t/5730) = .125
use nat logs
ln(2) = ln(.125)
 = 
 = -3
t = -3 * -5730
t = 17,190 yrs