N _ 2 O _ 5 decomposes to NO _ 2 and O _ 2 follows the first order...

$$\begin{array}{ll}{N}_{2}{O}_{5} \to 2{NO}_{2} + \dfrac{1}{2}{O}_{2} \\ t = 0 50 0 0 \\ t = 50mins50 - {p}_{1}2{p}_{1}\dfrac{{p}_{1}}{2} \\ t = 100mins50 - {p}_{2}2{p}_{2}\dfrac{{p}_{2}}{2}\end{array}$$

$50 - {p}_{1} + 2{p}_{1} + \dfrac{{p}_{1}}{2} = 87 .5$

$50 + \dfrac{3{p}_{1}}{2} = 87.5$

$\dfrac{3{p}_{1}}{2} = 37.5$

${p}_{1} = \dfrac{37.5 \times 2}{3} = 25$

$50$ minutes is half-life period.

For $100$ minutes ( half-life $2$)

$50 - {p}_{2} = 12 .5$

${p}_{2} = 37 .5 mmHg$

Total pressure at $100$ minutes.

$= 50 - {p}_{2} + 2{p}_{2} + \dfrac{{p}_{2}}{2}$

$50 + \dfrac{3{p}_{2}}{2} = 50 + \dfrac{3}{2} \times 37.5$

$= 50 + 56.25$

$= 106 .25 mmofHg$.