$${t = \frac{T_{1/2} \ln (1 + N_{Ar}/N_K)}{\ln 2} \\ = \frac{(1,25 \times 10^9\, \mathrm{a})[\ln (1 + 10,3)]}{\ln 2} \\ = \underline{\underline{4,37 \times 10^9\, \mathrm{a}}}}$$