$\begin{aligned} \frac{d N}{d t} &=\lambda N \\ \frac{d N}{d t} &=\frac{1}{T} \frac{6.023 \times 10^{23}}{12} \\ T &=\frac{6.023 \times 10^{23}}{12 \times 1.18 \times 10^{13}} \quad\left\{\frac{d N}{d t}=1.18 \times 10^{13}\right\} \\ &=42530 \times 10^{6} \mathrm{~min} \\ &=\frac{4253 \times 10^{6}}{60 \times 24 \times 365} \text { years } \quad\left\{\because \lambda=\frac{1}{T}\right\} \\ &=8092 \text { years } \\ & \approx 8100 \text { years } \end{aligned}$