$K\,\, = \,\,\frac{{2.303}}{t}\,\log \,\frac{a}{{(a\, - \,x)}}\,\,\,\, \Rightarrow \,\,\,\frac{{0.693}}{{20}}\,\, = \,\,\frac{{2.303}}{{40}}\,\log \,\frac{a}{{(a\, - \,x)}}$

$2\,\, \times \,\,0.3010\,\, = \,\,\log \,\frac{a}{{(a\, - x)}}\,\,\,\, \Rightarrow \,\,\,2\,\, \times \,\,\log \,2\,\, = \,\,\log \frac{a}{{(a\, - \,x)}}$

$\log \,\,{2^2}\, = \,\,\log \frac{a}{{(a\, - \,x)}}\,\,\,\, \Rightarrow \,\,\,\frac{a}{{(a\, - \,x)}}\,\, = \,\,4\,\,\,\, \Rightarrow \,\,\,\,\frac{{a\, - \,\,x}}{a}\,\, = \,\,\frac{1}{4}$