$20\% \,$ સંપૂર્ણ થવા માટે $\,{\text{K}}\, = \,\,\frac{{\text{1}}}{{{\text{500}}}}\,\, \times \,\,\frac{{20}}{{100\,\, \times \,\,80}}\,\,\,\,\,\,..........(1)$

$80\% $ પૂર્ણ થવા માટે ${\text{K}}\,\, = \,\,\frac{{\text{1}}}{{\text{t}}}\,\, \times \,\,\frac{{80}}{{100\,\, \times \,\,20}}\,\,\,\,\,\,............(2)$

$\frac{1}{{500}}\,\, \times \,\,\frac{{20}}{{100\,\, \times \,\,80}}\,\, = \,\,\frac{1}{t}\,\, \times \,\,\frac{{80}}{{100\,\, \times \,\,20}}$

$t\,\, = \,\,3000\,\,\sec .$