Well the first step is to identify what the maximum oxidation state is of the positively charged elements in each oxide. This is for the most part just something you have to memorize, or look up. So (a) is vanadium. What is the max oxidation state of vanadium?
Hello! Thank you for your answer. This is exactly the problem. If it were elements from the A families instead of the B, then I could easily solve it.
Vanadium's configuration is, for example, [Ar] 4s2 3d3. So it has 2 electrons on the valence shell, and it means it can lose its two last electrons to become stable like a noble gas. But where does +5 come from? I suppose that is the answer for what you have asked me, for the answer of the exercise is V2O5. O is -2, so V must be +5.
Is there some way to discover these values? Or is the only way by memorising?
edit: I have just noticed how it needs to lose 5 electrons to become stable. Sorry, I must have attention deficit. Is this the answer?
EDIT 2: YESSSS. Thank you very much. I just needed someone to guide my thoughts. I finally have learned it! It seems like it was exactly that: the metal must oxide and lose its electrons orbital by orbital from +1 to +9 to become exactly like a noble gas. Doing this and assigning the number of oxygens led me to the answers successfully!