Explanation

(b)
$t = \frac{2.303}{k}\log\frac{a}{a - x}$
Where, $k$=rate constant=$10^{- 3}s^{- 1}$
$a$=initial amount=100
$a - x$=amount left after time t=25
$t$=time to leave 25% reaction
$\therefore\ t = \frac{2.303}{10^{- 3}}\log\frac{100}{25}$
$= \frac{2.303}{10^{- 3}}\log 4$
$= \frac{2.303 \times 0.6020}{10^{- 3}}$
$= 1386s$