$\therefore $ Conc. after $20\,min$ (two half lives) $=\,2.5\,mol\,L^{-1}$
Now, $k\, = \,\frac{{0.693}}{{{t_{1/2}}}}\, = \,\frac{{0.693}}{{10\,\min }}$
or $0.0693\,min^{-1}$
$\therefore $ rate $=\,k\times $ [reactant]
$=\,0.0693\times 2.5\,\,mol\,L^{-1}\,min^{-1}$
$1$. $[A]$ $0.012$, $[B]$ $0.0351\rightarrow $ પ્રારંભિક દર $ = 0.10$
$2$. $[A]$ $0.024$, $[B]$ $0.070\rightarrow $ પ્રારંભિક દર $= 1.6$
$3$. $[A]$ $0.024$, $[B]$ $0.035\rightarrow $ પ્રારંભિક દર $ = 0.20$
$4$. $[A]$ $0.012$ , $[B]$ $0.070\rightarrow $ પ્રારંભિક દર $ = 0.80$