The balanced equation for the above reaction is
2 X + 5/2 O2 → X2O5
Now the mass of oxygen using law of conservation of mass = 3.6769 - 2.0769
Hence mass of Oxygen utilized in the reaction = 1.6 g
number of moles of oxygen = given mass / molar mass
Hence the number of moles of oxygen = 1.6/ 32 = 0.05 moles
As 5/2 moles of O2 reacts with 2 moles of X
Hence
5/2 mole of O2 → 2 moles of X
1 mole of O2 → 4/5 moles of X
0.05 moles of O2 reacts with → 4× 0.05/ 5 moles of X
Hence the number of moles of X present in the system = 0.04.