$R \rightarrow P$
$-x x=\frac{2}{3} a $
$a-\frac{2}{3} a$
According to the half-life of a first order reaction,
$t=\frac{2.303}{k} \log \frac{ a }{ a - x }$
$=\frac{2.303}{4.3 \times 10^{-4}} \log \frac{ a }{ a / 3}$
$=\frac{2.303}{4.3 \times 10^{-4}} \log 3$
$=2.5 \times 10^{3} \,sec$
$A\,\xrightarrow{{{K_1}}}\,B,$ સક્રિયકરણ ઊર્જા ; $Ea_1$
$A\,\xrightarrow{{{K_2}}}\,C,$ સક્રિયકરણ ઊર્જા $Ea_2$