8)                     

Oxidation number of O in Fe₃O₄ = (-2) x 4 = -8
Oxidation number of Fe = +8
Possible number of Fe²⁺ and Fe³⁺ atoms = 8 = (3 x 2) + (2)
So there are 2 Fe3+ atoms and 1 Fe2+ atom.

Fraction = ⅓