200ml of ${\mathrm{O}}_2$ gas maintained at 700 mm pressure and 250ml of ${\mathrm{N}}_2$ gas maintained at 720mm pressure are put together in one litre flask. If the temperature is kept constant, the final pressure of the mixture in mm is

According to Boyle's law 

Final volume=1 L

Final volume =1000 ml

PV= constant

if the temperature is kept constant and final pressure of the mixture

$\mathrm{P}=\frac{{\mathrm{P}}_1{\mathrm{V}}_1+{\mathrm{P}}_2{\mathrm{V}}_2}{{\mathrm{V}}_1+{\mathrm{V}}_2}$

$\mathrm{P}=\frac{{\mathrm{P}}_1{\mathrm{V}}_1+{\mathrm{P}}_2{\mathrm{V}}_2}{{\mathrm{V}}_{\mathrm{final}}}$

$\mathrm{P}=\frac{700\times 200+720\times 250}{1000}$

P=320 mm

Correct option is B.