Answer
154 mmHg
Work Step by Step
$P_{total}=P(N_{2})+P(Ar)+P(CO_{2})+P(H_{2}O)+P(O_{2})$
$\implies 740.\, mmHg= 575\,mmHg+6.9\, mmHg+0.2\, mmHg+4.0\, mmHg+P(O_{2})$
$\implies P(O_{2})=740.\, mmHg-575\,mmHg-6.9\, mmHg-0.2\, mmHg-4.0\, mmHg=154\, mmHg$
