This problem can be solved by Dalton's law of partial pressures which states that in a mixture of non-reacting gases, the total pressure exerted is equal to the sum of the partial pressures of the individual gases.
Ptotal = P₁ + P₂ + P₃ + ..... + Pn
Herein, Ptotal = P(Ne) + P(Ar) + P(He)
Ptotal = 1.25 atm, P(Ne) = 0.68 atm and P(Ar) = 0.35
So, P(He) = Ptotal - P(Ne) - P(Ar) = 1.25 - 0.68 - 0.35 = 0.22 atm