The fertilizer contains 7.90 % P by mass.
Assume you have 100 g of fertilizer.
Mass of P₄O₁₀ = 100 g fertilizer × 18.1 g P₄O₁₀/100 g fertilizer = 18.1 g P₄O₁₀
M_r of P₄O₁₀ = 4 × 30.97 + 10× 16.00 = 123.88 + 160.00 = 283.88
Mass of P = 18.1 g P₄O₁₀ × (123.88 g P/283.88 g P₄O₁₀) = 7.90 g P
Mass % P = mass/total mass × 100 % = 7.90 g /100 g × 100 % = 7.90 %