Answer:
∠ B = 40° , ∠ C = 20°
Explanation:
the 3 angles in the triangle sum to 180° , that is
∠ A + ∠ B + ∠ C = 180° ( substitute ∠ B = 2 ∠C )
∠ A + 2 ∠ C + ∠ C = 180°
120° + 3 ∠ C = 180° ( subtract 120° from both sides )
3 ∠ C = 60° ( divide both sides by 3 )
∠ C = 20°
∠ B = 2 × ∠ C = 2 × 20° = 40°