This chemical equation would hold only with an excess of oxygen and with something (usually a lot of nitrogen) that limits the temperature and at a reasonable pressure.
Here I agree that the reaction lacks oxygen. This does NOT mean that only a part of the propane is burnt and this part follows the equation. With such an oxygen/fuel mismatch, expect all the propane to be affected, but not form these products, or not only these.
So among the products, you may also have H2, CO, C. The temperature must tell you which ones you can have, which one takes the oxygen to the others, and then the available oxygen tells the amount possible for each product.
Or you do it with chemical equilibria CO/CO2, H2/H2O, and so on - or between the elements and the possible products. But at 120°C the "equilibria" will mean: everything for one, nothing for the other.
The question implicitly suggests that the products are at equilibrium, and I dislike that. Don't ask too many question to get a good mark, but please keep in mind for real life that a combustion is not always at equilibrium. For instance, once soot is formed, it won't burn even if you add oxygen, even at high temperature.