Let  $a=1$,

${{K}_{2}}=\frac{1}{t}\frac{x}{(1-x)}$

$=\frac{1}{3000}\left( \frac{0.6}{1-0.6} \right)$

$=\frac{1}{3000}\times \frac{0.6}{0.4}=\frac{1}{2000}$

$20\%$ પૂરી થવા માટે 

$\therefore \,\text{t}=\frac{\text{1}}{{{\text{k}}_{\text{2}}}}\frac{x}{a(a-x)}$

$=2000\times \frac{0.20}{0.80}$

$=500\,\sec$