What is the actual amount of phosphorous in a fertilizer with a npk ratio of 15-15-20?

NPK ratio system is a conventional shorthand for the ratio of the nitrogen N), phosphorus (P) and potassium (K) in a fertilizer. It actually shows amount of P₂O₅.
If we use 100 kg of P₂O₅:
m(P₂O₅) = 0,15 · 100 kg.
m(P₂O₅) = 15 kg.
m(P₂O₅) : M(P₂O₅) = m(P) : 2M(P).
15 kg : 142 = m(P) : 62.
m(P) = 6,55 kg.
ω(P) = 6,55 kg ÷ 100 kg · 100% = 6,55%.

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