This is a numerical-answer question. The correct answer is shown below.
[Solution by Dr. Abubacker] First order: $\ln\frac{P_0}{P_A}=kt=4.606=2\ln10\Rightarrow P_A=\frac{0.6}{100}=0.006$ atm. If $2x'$ atm of N₂O₅ decomposes, the total pressure is $P_0-2x'+2x'+x'=P_0+x'$. Here $2x'=0.6-0.006=0.594$, so $x'=0.297$ and $P_t=0.6+0.297=0.897$ atm $=897\times10^{-3}$ atm. Answer: 897.