$K= \frac{{2.303}}{t}\,\log \left( {\frac{a}{{a\,\, - {x_1}}}} \right) $
$\Rightarrow \,\,K= \frac{{2.303}}{5}\,\log \,\frac{{100}}{{80}} = \frac{{2.303}}{5}\,\log \,\frac{5}{4}$
$K = \frac{{2.303}}{5}[\log \,5 - \log \,4] $
$\Rightarrow \,K = \,\,\frac{{2.303}}{5}[0.6989 - 0.6020]$
$\Rightarrow K = \frac{{2.303}}{5}[0.0969] = 0.0446$
હવે , ${x_2} = 60$
$K =\frac{{2.303}}{t}\,\log \,\frac{a}{{a -{x_2}}}$
$t =\frac{{2.303}}{{0.0446}}\,\log \,\frac{{100}}{{40}} = \frac{{2.303}}{{0.0446}}[\log \,5 - \log \,\,2]$
$ = \,\frac{{2.303}}{{0.0446}}0.3979 = 20.55\,\min $