Two radioactive materials $ X_1\: and\: X_2$ have decay constants $ 5\lambda\: and\: \lambda$ respectively. If initially they have the same number of nuclei, then the ratio of the number of nuclei of $ X_1$ to that of $ X_2$ will be $ \large\frac{1}{e}$ after a time :

Question

$\begin {array} {1 1} (1)\;\large\frac{e}{\lambda} & \quad (2)\;\lambda \\ (3)\;\large\frac{1}{2}\lambda & \quad (4)\;\large\frac{1}{4\lambda} \end {array}$