Answer: The partial pressure of oxygen is 180 mm Hg
Explanation:
According to Dalton's law, the total pressure is the sum of individual pressures.
[tex]p_{total}=p_{N_2}+p_{CO_2}+p_{He}+p_{O_2}[/tex]
Given : [tex]p_{total}[/tex] =total pressure of gases = 725 mm Hg
[tex]p_{N_2}[/tex] = partial pressure of nitrogen  = 355 mm Hg
[tex]p_{CO_2}[/tex] = partial pressure of carbon dioxide  = 175 mm Hg
[tex]p_{He}[/tex] = partial pressure of helium  = 15 mm Hg
[tex]p_{O_2}[/tex] = partial pressure of oxygen  = ?
putting in the values we get:
[tex]725=355+175+15+p_{O_2}[/tex]
[tex]p_{O_2}=180mmHg[/tex]
The partial pressure of oxygen is 180 mm Hg