Nucleus $\mathrm{A}$ decays into $\mathrm{B}$ with a decay constant ${\lambda }_1$ and $\mathrm{B}$ further decays into $\mathrm{C}$ with a decay constant ${\lambda }_2$. Initially, at $t=0$, the number of nuclei of $\mathrm{A}$ and $\mathrm{B}$ were $3N_0$ and $N_0$ respectively. If at t = t₀ number of nuclei of $\mathrm{B}$ becomes constant and equal to $2N_0$, then