Answer
900 mmHg.
Work Step by Step
Partial pressure of argon gas is
$P_{Ar}=0.25\,atm= 0.25\,atm\times\frac{760\, mmHg}{1\,atm}=190\, mmHg$
Partial pressure of helium gas is
$P_{He}= 350\, mmHg$
Partial pressure of nitrogen gas is
$P_{N_{2}}=360\,torr=360\,torr\times\frac{1\, mmHg}{1\,torr}$
$=360\, mmHg$
Using Dalton's law, we have, total pressure
$P_{total}=P_{Ar}+P_{He}+P_{N_{2}}$
$=190\, mmHg+ 350\, mmHg+360\, mmHg$
$=900\, mmHg$
