In the case of nitrogen, volume increases, its pressure must decrease. Let the new pressure be PN2
PN2=250×200300=166.6 mm (Applying Boyle's law)
In the case of oxygen, volume decreases, its pressure must increase. Let the new pressure be PO2
PO2=300×350300=350 mm (Applying Boyle's law)
Total pressure =PN2+PO2=(1666.6+350)=516.6 mm