Number of masses will remain constant
$\begin{aligned}
& \mathrm{n}1+\mathrm{n}2=\mathrm{n}{\mathrm{f}} \
& \frac{\mathrm{P}1 \mathrm{V}_1}{\mathrm{RT}_1}+\frac{\mathrm{P}_2 \mathrm{V}2}{\mathrm{RT}2}=\frac{\mathrm{P}{\mathrm{f}} \mathrm{~V}{\mathrm{f}}}{\mathrm{RT}{\mathrm{f}}} \
& \frac{8 \times 2 \mathrm{~V}}{\mathrm{R} \times 1000}+\frac{7 \times \mathrm{V}}{\mathrm{R} \times 500}=\frac{\mathrm{P}{\mathrm{f}}(3 \mathrm{~V})}{\mathrm{R} \times 600} \
& \frac{16}{1000}+\frac{14}{1000}=\frac{\mathrm{P}_{\mathrm{f}}}{\mathrm{R} \times 600} \
& \frac{30}{1000}=\frac{\mathrm{P}{\mathrm{f}}}{200} \
& \mathrm{P}{\mathrm{f}}=6 \mathrm{kPa}
\end{aligned}$