$\text { Initial moles } {a} \times \frac{50}{100} \quad\quad 0 \quad {a} \times \frac{50}{100} \quad0 \quad\quad 0$

$\text { Final moles }\quad 0 \quad 0.25 {a} \quad\quad\quad0 \quad\quad 0.5 {a} \quad\quad 0.5 {a}$

Now, $i=\frac{\text { final moles }}{\text { initial moles }}=\frac{0.25 a+0.5 a+0.5 a}{0.5 a+0.5 a}$

$=1.25=125 \times 10^{-2}$