Сумма углов треугольника равна 180°
∠A+∠B+∠C = 180°
∠B = 20°, ∠A = ∠C+30°
∠C+∠C+30° = 180°-20° = 160°
2∠C = 160°-30° = 130° ⇒ ∠C = 130°/2 = 65°
∠A = ∠C+30° ⇒ ∠A = 65°+30° = 95°
∠A = 95°
∠B = 20°
∠C = 65°