A radioactive substance X decays into another radioactive substance Y. Initially only X was present. ${\mathrm{\lambda }}_{\mathrm{x}}$ and ${\mathrm{\lambda }}_{\mathrm{y}}$ are the disintegration constants of X and Y. ${\mathrm{N}}_{\mathrm{x}}$ and ${\mathrm{N}}_{\mathrm{y}}$ are the number of nuclei of X and Y at any time t. Number of nuclei ${\mathrm{N}}_{\mathrm{y}}$ will be maximum when: