$$\begin{gathered} \mathrm{A}\left(\mathrm{g}\right)\stackrel{}{\rightleftharpoons }2\mathrm{B}\left(\mathrm{g}\right)+\mathrm{C}\left(\mathrm{g}\right) \\ \mathrm{intial} 450 0 0 \\ \mathrm{at} \mathrm{time}=\mathrm{t} 450-\mathrm{p} 2\mathrm{p} \mathrm{p} \end{gathered}$$

$(450 – \mathrm{p}) + \left(2\mathrm{p}\right) + \left(\mathrm{p}\right) = 720$

$450 + 2\mathrm{p} = 720$

$2\mathrm{p}= 270$

$\mathrm{p}= 135$

Fraction of $\mathrm{A}\left(\mathrm{g}\right)$ decomposed$=\frac{\mathrm{Amount} \mathrm{of} \mathrm{A} \mathrm{decomposed}}{\mathrm{Initial} \mathrm{amount}}$  $=\frac{135}{450}=0.3$

The fraction $\mathrm{A}\left(\mathrm{g}\right)$ decomposed = $3\times {10}^{-1}$

$\Rightarrow \mathrm{x}=3$