હવે , $k = \,\,\frac{{2.303}}{t}\,\,\log \,\,\frac{{100}}{{20}}\,\,\,\,\,\,\,\,[\,\,a\,\, = \,\,100,\,\,x\,\, = \,\,80,\,\,\,a\, - \,\,x\,\, = \,\,20]$
$\therefore \,\,\frac{{0.693}}{{69.3}}\,\, = \,\,\frac{{2.303}}{t}\,\,\log \,\,5\,\,\,\,\,\,\therefore \,\,t\,\, = \,\,160.97\,\min$