Radioactive material 'A' has decay constant $8 \lambda$ and material 'B' has decay constant $\lambda$. Initially they have same number of nuclei. After what time, the ratio of number of nuclei of material 'B' to that 'A' will be $\large\frac{1}{e}$?

Question

$\begin{array}{1 1} (1) \large\frac{1}{\lambda}\\ (2) \large\frac{1}{7\lambda} \\ (3) \large\frac{1}{8 \lambda} \\ (4) \large\frac{1}{9 \lambda} \end{array}$