Discrepancies in the experimental values of the molar mass of nitrogen provided some of the first evidence

Question:

Discrepancies in the experimental values of the molar mass of nitrogen provided some of the first evidence for the existence of the noble gases. If pure nitrogen is collected from the decomposition of ammonium nitrite

Its measured molar mass is 28.01. If O₂, CO₂, and H₂O are removed from air, the remaining gas has an average molar mass of 28.15. Assuming this discrepancy is solely a result of contamination with argon (atomic mass = 39.95), calculate the ratio of moles of Ar to moles of N₂ in air.