To do this question,
The (V) sign denotes that…. Phosphorus has a POSITIVE oxidation state of (+5)
A little note : for things like dichromate(VI) and Permanganate (VII), the VI and VII in both cases refer to the positive oxidation of the chromium and manganese ion respectively.
The reason why they are incredible oxidising agents is because the oxidation state is so high! Cr(6+) and Mn(7+) in Cr2O7 and MnO4, therefore in reactions usually these elements get reduced to green Cr(3+) and colourless Mn(2+) respectively.
Next, other than phosphorus in phosphorus (V) oxide, there is oxygen J
So we know that oxygen has a NEGATIVE oxidation state of (-2)
Since phosphorus (V) oxide is an uncharged compound, unlike say OH(-),
The NEGATIVE and POSITIVE oxidation state must BALANCE OUT.
Searching for the Lowest Common Multiple of 10 for our oxygen (2-) and phosphorus (5+), we arrive at P2O5