For the first-order reaction $\left(C=C_0{e}^{-k_{1}t}\right)$ and ${\mathrm{T}}_{av}=k_1^{-1}$ After two average lives concentration of the reactant is reduced to:

Given,
first order reaction $C=C_0{e}^{-1k_{1}t}$

                   and $T_{aw}=K_1^{-1}$

$\begin{array}{l}C=C_0{c}^{-k2T_{\omega }}\\ \Rightarrow C=C_0{e}^{-k/k_1}\times 2\\ \Rightarrow C=C_0{e}^{-2}\\ \Rightarrow \frac{C}{C_0}=e^{-2}.\\ \Rightarrow \frac{C}{C_0}\times 100=\frac{100}{e^2}\mathrm{\%}\end{array}$