Oxidation Numbers Help?

a.
P₄ is an element.
The oxidation number of P in P₄ = 0

b.
PF₃ is a compound, and thus the sum of oxidation numbers = 0
The oxidation number of F in PF₃ = -1 (fixed)
The oxidation number of P in PF₃ = 0 - 3(-1) = +3

c.
PH₃ is a compound, and thus the sum of oxidation numbers = 0
The oxidation number of H in PH₃ = +1 (fixed)
The oxidation number of P in PH₃ = 0 - 3(+1) = -3

d.
P₂O₃ is a compound, and thus the sum of oxidation numbers = 0
The oxidation number of O in P₂O₃ = -2 (fixed)
The oxidation number of P in P₂O₃ = [0 - 3(-2)] / 2 = +3

e.
H₃PO₄ is a compound, and thus the sum of oxidation numbers = 0
The oxidation number of H in H₃PO₄ = +1 (fixed)
The oxidation number of O in H₃PO₄ = -2 (fixed)
The oxidation number of P in H₃PO₄ = [0 - 3(+1) - 4(-2)] = +5