Q. Two radioactive materials X₁  and X₂ contain same number of nuclei . If 6 $\lambda s^{-1}$ and 4$\lambda s^{-1}$ are the decay constant ​of X₁  and X₂   respectively , the ratio of number of nuclei undecayed of X₁   to that of  X₂   will be 1/e after a time
(1) $\frac{1}{2\lambda }s$              (2) $\frac{1}{10\lambda }s$             (3) $\frac{1}{5\lambda }s$                (4) $\frac{1}{8\lambda }s$