∠m₁n₁ + ∠m₁n₂ = 240°.
∠m₁n₁ + ∠mn = 180°.
∠mn = x.
∠m₁n₂ = 240° - ∠m₁n₁.
∠m₁n₁ + 240° - ∠m₁n₁ = 180°.
∠mn = 360° - 240° = 120°.
Ответ: ∠mn = 120°.