$log$નો ગુણધર્મ $\ln \left(\frac{{x}}{{y}}\right)=\ln {x}-\ln {y}$
${t}=0\quad \quad \quad \quad 1 \text { mole } \quad 0$
${t}=100 {~min}\quad 1-{x} \quad \quad 2 {x}$
$\quad \quad \quad \quad \quad =0.9 {~mol} \quad =0.2 {~mol}$
Now, $t=\frac{t_{1 / 2}}{\ln 2} \times \frac{\left[A_{0}\right]}{\left[A_{t}\right]}$
$100=\frac{{t}_{1 / 2}}{\ln 2} \times \ln \frac{1}{0.9} \Rightarrow {t}_{1 / 2}=690\, {~min} . \quad$ (Taking $\left.\ln 3=1.11\right)$
Answer is $700 .$ (Nearest integer).
$(\log \,4 = 0.60,\, \log \,5 = 0.69)$