Two radioactive materials $\mathrm{A}$ and $\mathrm{B}$ have decay constants $10\mathrm{\lambda }$ and $\mathrm{\lambda }$ , respectively. If initially they have the same number of nuclei, then the ratio of the number of nuclei of $\mathrm{A}$ to that of $\mathrm{B}$ will be $1/\mathrm{e}$ after a time: