Step-by-step explanation:
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In a salt, we must take into account that the sum of the oxidation numbers is equal to zero since the molecule is stable.
In this case the molecule is a salt and is (NH4) 3PO4
N = -3
H = + 1 (there are four atoms in total is +4)
(NH4) 3 = (- 3 + 4) * 3 = + 3
P = + 5
O = -2 (since there are 4 atoms in total it is -8)
PO4 = + 5-8 = -3
The total is: -3 + 3 = 0
We verify that the charges give a total of zero, so that our calculation is well done.