For the first order reaction $\mathrm{A}\to 2 \mathrm{B},1$ mole of reactant A gives $0.2$ moles of $B$ after $100$ minutes. The half life of the reaction is ...... min. (Round off to the nearest integer).

[Use $:\ln 2=0.69,\ln 10=2.3$

Properties of logarithms $:\ln x^y=y\ln x$

$\left.\ln \left(\frac{x}{y}\right)=\ln x-\ln y\right]$

(Round off to the nearest integer)

Hence, the correct answer is 655.