If carbon-14 has a half-life of 5730 years what age is a wooden object with carbon-14 activity of 10 cpm compared to 40 cpm

k = ln2 / t1/2 = 1.21*10^-4 yr-1
t = -ln ([A]t/[A]o) / k = -ln (10/40) / 1.21*10^-4 = 11,450 yr