$t = \frac{{2.303}}{K}\log \frac{a}{{a - x}}$
$ K= \frac{{0.693}}{{t_{1/2}}}= \frac{{2.303}}{t}\log \left( {\frac{{100}}{{100 - 90}}} \right)$
$\therefore \,\,\frac{{0.693}}{{120}} = \frac{{2.303}}{t}\log \,10\,$
$\,\,\therefore \,\,t = \frac{{120 \times 2.303}}{{0.693}} = 398.8\,$ મિનિટ
$1$. $[A]$ $0.1$, $[B]$ $0.1 - $ પ્રારંભિક દર $ \rightarrow 7.5 \times 10^{-3}$
$2$. $[A]$ $0.3$, $[B]$ $0.2 -$ પ્રારંભિક દર $ \rightarrow 9.0 \times 10^{-2}$
$3$. $[A]$ $0.3$, $[B]$ $0.4 -$ પ્રારંભિક દર $ \rightarrow 3.6 \times 10^{-1}$
$4$. $[A]$ $0.4$, $[B]$ $0.1 -$ પ્રારંભિક દર $ \rightarrow 3.0 \times 10^{-2}$