Atoms And Nuclei

Two radioactive materials \(X_1\) and \(X_2\) 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 \(\mathrm{X}_2\) is \(\frac{1}{\mathrm{e}}\). Then \(\mathrm{t}\) is equal to