I have an assignment (I quote it word by word):
Determine the oxidation number of carbon in (A) ethanol and (B) ethanal.
Answers are (A) -2 and (B) -1.
I reviewed similar question at SE: Oxidation of Carbons
and I found first answer to be clear and links given in this answer to be useful.
I also found a good Khan Academy video, which basically confirms all in this answer
Now, here is my problem: There are two carbons in ethanol. Oxidation number of carbon which is connected to three hydrogens is $-3$; oxidation number of carbon which is connected to two hydrogens and one oxygen is $-1$. How do we get answer $-2$ out of it then? (Do we add $-3 + (-1)$ and then divide by 2?)
And again there are two carbons in ethanal. Oxidation number of carbon which is connected to three hydrogens is $-3$; oxidation number of carbon which is connected to one hydrogen and one oxygen (but with double bond) is $+1$. How do we get answer $-1$ out of it? (Do we add $-3 + 1$ and divide it by 2?)
Overall, my question does not seem to concentrate on one of the carbons present in ethanol (or ethanal). And answers given look like there is oxidation number of first carbon added to oxidation number of second carbon and then divided by 2. This situation seems to be true for ethanol, and again the same happens for ethanal. Where can I read about adding two oxidation states and then calculating average (I have never heard of it)? Or maybe these calculations done in some other way? Thank you very much in advance for answering.