Given,

The one forth of gas is released then the number of moles of a gas is $n_2=\frac{3}{4}n$

Volume of a gas is $V=$constant

Initial pressure of a gas is $P_1=600 \mathrm{KPa}$

Temperature of a gas $T_1=300 \mathrm{K} \& T_2=600 \mathrm{K}$

Using ideal gas equation,

$\frac{P_1{V}_1}{n_1{T}_1}=\frac{{\displaystyle P_2{V}_2}}{{\displaystyle n_2{T}_2}}$ $\because R$ is gas constant 

The final pressure of the gas is given by

$P_2=\frac{n_2{V}_1{T}_2}{n_1{V}_1{T}_1}=\frac{3}{4}\times \frac{600}{300}\times 600$

$\Rightarrow P_2=900 \mathrm{KPa}$