Answer is: the new pressure on the gas is 0.25 atm or 25.33 kPa.
Boyle's Law: the pressure volume law - volume of a given amount of gas held varies inversely with the applied pressure when the temperature and mass are constant.
p₁V₁ = p₂V₂.
1 atm · 5.00 L = p₂ · 20.0 L.
p₂ = 1 atm · 5 L ÷ 20.0 L.
p₂ = 0.25 atm, but same amount of helium.