Answer :

3) P4O10 and P2O5
The mass of the P4O10 divided by the mass of the P2O5=2, and if you multiply the number of atoms in the P2O5 by 2, you get the P4O10, thus P2O5 is its empirical formula.
3 because 4 divided by 2 is 2 (P2) and 10 divided by 2 is 5 (O5) which reduces P4O10 to P2O5