40度
應該不會錯

來個做法

作平行四邊形BDFC，DE = EC = AD
因為∠FCA =∠DAE =∠DEA，DE = EC，DE = CF
所以∆ADE全等於∆FEC，故CE = EF
注意DE = EC = AD = BC = DF = EF，∆DEF為正三角形
設∠ABC = X，則∠DAE = 2X，∠ADE = 180°－4X，∠ADF = X
∠ADF +∠ADE = 60°，即X =∠ABC = X = 40°