●×2 = 40°+〇×2
よって、●=20°+〇
∠BEC=180 - (〇 + ● + ∠ACB) = 180 - ｛〇 + ● + (180 - ●×2)｝
= 180 - (〇 + 180 - ●) = 180 - ｛〇 + 180 - (20 + 〇)｝ = 180 -160 = 20