${\mathrm{N}}_2$ is found in a litre flask under 100 kPa pressure and $O_2$ is found in another 3 litre flask under 320 kPa pressure. If the two flasks are connected, the resultant pressure is

Total volume of two flasks = 1 + 3 = 4

If $P_1$ the pressure of gas $N_2$ in the mixture of $N_2$ and $O_2$, then

P = 100 kPa, ${\mathrm{P}}_1=?,\mathrm{V}=1\mathrm{litre},{\mathrm{V}}_1=4\mathrm{litre}$

Applying Boyle's law $\mathrm{PV}={\mathrm{P}}_1{\mathrm{V}}_1$

$100\times 1={\mathrm{P}}_1\times 4;{\mathrm{P}}_1=25$

If ${\mathrm{P}}_2$ is the pressure of ${\mathrm{O}}_2$ gas in the mixture of ${\mathrm{O}}_2$ and ${\mathrm{N}}_2$

then, $320\times 3={\mathrm{P}}_2\times 4;{\mathrm{P}}_2=240$

Hence, total pressure $\mathrm{P}={\mathrm{P}}_1+{\mathrm{P}}_2=25+240=264\mathrm{kPa}$