Answer: 19kpa
Step-by-step explanation:
The total pressure of a mixture of gases is the sum of the partial pressure exacted by the individual gases.
P total = P X + P Y
From the question above:
P total = P .nitrogen + P. oxygen
123.5kpa = 104.5kpa + P. Oxygen
P. Oxygen = 123.5kpa - 104.5kpa
= 19kpa