Substance remained after time $\mathrm{t}$,
$\mathrm{N}={\mathrm{N}}_0{\mathrm{e}}^{-\mathrm{\lambda t}}$

Mean life of radioactive sample is, ${\mathrm{t}}_{\mathrm{mean}}=\frac{1}{\mathrm{\lambda }}$
$\therefore$ Substance remained after mean life
$\mathrm{N}={\mathrm{N}}_0{\mathrm{e}}^{-1}$
$\mathrm{N}=\left(\text{0.37}\right){\mathrm{N}}_0$
$\therefore$ Substance disintegrated
${\mathrm{N}}_0-\mathrm{N}=0.63{\mathrm{N}}_0$
i.e. $63\mathrm{\%}$, nearly $\frac{2}{3}$.