Answer is: 180 mL of 7% vinegar and 60 mL of 11% vinegar.
ω₁ = 7% ÷ 100% = 0.07.
ω₂ =
11% ÷ 100% = 0.11.
ω₃ =
8% ÷ 100% = 0.08.
V₁ = ?.
V₂ =
?.
V₃ =
V₁ + V₂.
V₃ = 240 mL.
V₁ = 240 mL - V₂.
ω₁ ·
V₁ + ω₂ ·V₂ =
ω₃ · V₃.
0.07 · (240 mL - V₂) + 0.11 · V₂ = 0.08 · 240 mL.
16.8 mL - 0.07V₂ + 0.11V₂ = 19.2 mL.
0.04V₂ = 2.4 mL.
V₂ = 2.4 mL ÷ 0.04.
V₂ = 60 mL.
V₁ = 240 mL - 60 mL.
V₁ = 180 mL.