Answer:
Search by reactants (P 2O 5, H 2O) and by products (H 3PO 4)
H2O + P2O5 → H3PO4
H2O + HNO3 + P2O5 → H3PO4 + N2O5
If it is just ratio you must just put the mass number on the either side of the symbol ‘:’ and cancel but the same process is not used to find simplest ratio