a
${{t}_{1/2}}=6.93$, $a{\mkern 1mu} {\mkern 1mu} = {\mkern 1mu} {\mkern 1mu} 100,$ $x{\mkern 1mu} {\mkern 1mu} = {\mkern 1mu} {\mkern 1mu} 99,{\mkern 1mu} $ $a{\mkern 1mu} - {\mkern 1mu} x{\mkern 1mu} {\mkern 1mu} = {\mkern 1mu} {\mkern 1mu} 1$
${K_1}\, = \,\,\frac{{2.303}}{t}\log \frac{a}{{a\, - \,\,x}}$
$\Rightarrow \,\frac{{0.693}}{{6.93}}\, = \,\frac{{2.303}}{t}\log \frac{{100}}{1}$
$t = 46.06\,\min$