when the total pressure equal sum of all the partial pressure with the same units.
and when we have total pressure = 780 mmHg = 780 torr
So total pressure = P(CO2) + P(O2)
by substitution:
780 torr = 560 torr + P(O2)
∴P(O2) = 780 torr - 560 torr
= 220 torr
