I think you might find your answer here https://answers.yahoo.com/question/index?qid=20121101101721AAeZyHE
Answer:
Age ≅ 7500 years
Explanation:
All radioactive decay is 1st order kinetics and described by the expression
A = A₀e^-kt => t = ln(A/A₀) / -k
k = 0.693 / t(half life) = (0.693 / 5730)yrs⁻¹ = 1.21 x 10⁻⁴ yrs⁻¹
t = Age = [ln(0.103/0.255) / - 1.21 x 10⁻⁴] yrs = 7500 years
