First of all, as you seen the gases are noble which means that will not react with each other and in this case each gas create individual pressure.
P[tex]_{T}[/tex]= total pressure
P[tex]_{Ne}[/tex] = pressure of neon
P[tex]_{Ar}[/tex] = pressure of argon
P[tex]_{He}[/tex] = pressure of helium {which is required}
P[tex]_{T}[/tex] = P[tex]_{Ne}[/tex] + P[tex]_{Ar}[/tex] + P[tex]_{He}[/tex]
1.25 = 0.68 + 0.35 + P[tex]_{He}[/tex]
P[tex]_{He}[/tex] = 1.25 - [0.68 + 0.35] = 0.22 atm