How to calculate % free SO3 in given Oleum sample ?

CALCULATIONS OF FREE SO₃ IN (X%) OLEUM SAMPLE :
Given - X %  labelled 100 gm Oleum,it means  (X-100) gm water required to destroyed all free SO₃present in 100 gm Oleum.
                                               W_H2O= (X-100) gm

BY STOICHIOMETRIC CALCULATION:

Weight of SO₃ = nSO3 X Molecular wt SO₃

Weight of H₂SO₄ = nH₂SO₄ X Molecular wt of H₂SO₄

 

thus % (Free) SO₃ present in 100 gm 109% labelled Oleum Sample is