Explanation

(b)
For Ist order reaction
$k = \frac{2.303}{t}\log\frac{\lbrack A\rbrack_{0}}{\lbrack A\rbrack}$
If $\lbrack A\rbrack_{0} = a,\ \lbrack A\rbrack = a - \frac{a\ \times \ 20}{100} = 0.80a$
$t = 10\ \min$
$k = \frac{2.303}{10}\log\frac{a}{0.80\ a} = \frac{2.303}{10} \times 0.0969$
$\ = 0.0223$
If $\lbrack A\rbrack_{0} = a,\ \lbrack A\rbrack = a - \frac{a\ \times 75}{100} = 0.25\ a$
$t = \frac{2.303}{k}\log\frac{\lbrack A\rbrack_{0}}{\lbrack A\rbrack}$
$= \frac{2.303}{0.0223}\log\frac{a}{0.25\ a}$
$= \frac{2.303}{0.0223} \times 0.6021$
$= 62.18\ \min$