Here's why:

* Oxidation numbers represent the hypothetical charge an atom would have if all bonds were ionic.

* In a neutral molecule, the sum of the oxidation numbers of all atoms must equal zero.

* Hydrogen typically has an oxidation number of +1.

* Carbon has a variable oxidation number, but in hydrocarbons, it's typically -3.

Let's calculate the oxidation number of carbon in propane:

* 3 carbon atoms * x (oxidation number of carbon) + 8 hydrogen atoms * (+1) = 0

* 3x + 8 = 0

* 3x = -8

* x = -8/3

Since we can't have fractional oxidation numbers, we can say that the average oxidation number of carbon in propane is -3.

Important Note: While the average oxidation number of carbon in propane is -3, this doesn't mean each carbon atom individually has a -3 oxidation number. The oxidation numbers are assigned based on electronegativity differences, and the bonds in propane have some covalent character.