The partial pressure of the third gas (Helium) is 0.22 atm
calculation
Total pressure= partial pressure for neon + partial pressure for argon + partial pressure for helium
partial pressure for neon= 0.68 atm
partial pressure for argon = 0.35 atm
partial pressure for helium = ? atm
make the partial pressure of helium the subject of the formula
Partial pressure of Helium= total pressure - ( partial pressure of neon +partial pressure for Helium )
= 1.25 atm - (0.68 +0.35 ) =0.22 atm