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Physics

# 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 :

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

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