Find the average oxidation number of (a) iron in Fe₃O₄; (b) sulfur in Na₂S₄O₆; (c) carbon in C₃O₂…

Step 1. Iron. $3x + 4(-2) = 0$, so $x = \dfrac{8}{3} = 2.67$.

Step 2. Sulfur. $2(+1) + 4x + 6(-2) = 0$, so $4x = 10$ and $x = 2.5$.

Step 3. Carbon. $3x + 2(-2) = 0$, so $x = \dfrac{4}{3} = 1.33$.

Step 4. The average. In $\text{Fe}_3\text{O}_4$ the three iron atoms are not alike. One is $+2$ and two are $+3$, because $2 + 3 + 3 = 8$.