How many liters of 15 acid and 33 acid should be mixed to make 40 liters of 21 acid solution answer key?

Answer is: 26.67 liters of 15% acid and 13.33 liters of 33% acid are needed.
ω₁(acid) = 15% = 0,15.
ω₂(acid) = 33% = 0,33.
V₁(acid) = ?
V₂(acid) = ?
V₃(acid) = 40l.
ω₃(acid) = 21% = 0,21
1) V₁ + V₂ = 40l, V₁ = 40 - V₂.
2) 0,15·V₁ + 0,33·V₂ ÷ 40 = 0,21
0,15·(40 - V₂) + 0,33·V₂ ÷ 40 = 0,21
6 - 0,15V₂ + 0,33V₂ ÷ 40 = 0,21.
0,18V₂ = 2,4, V₂ = 13,33l.
V₁ = 40 - 13,33 = 26,67l.