Assign oxidation number for all the elements in each of the compound
KMnO4
(NH4)2HPO4
P4O6
KMnO4:
K: +1
Mn: +7
O: -2
(NH4)2HPO4:
N: -3 (because each ammonium group has a total charge of +1 and there are 2 of them)
H: +1
P: +5
O: -2
P4O6:
P: +3
O: -2