Physics - Modern Physics Question with Solution | TestHub

Two radioactive materials ${\mathrm{X}}_1$ and ${\mathrm{X}}_2$ have decay constants ' $5\lambda$ ' and ' $\lambda$ ' respectively. Initially, they have the same number of nuclei. After time ' $t$ ', the ratio of number of nuclei of $X_1$ to that of $X_2$ is $\frac{1}{e}$. Then $t$ is equal to