1994 AMC 8 第 7 题

In triangle $ABE$, the angles at $A$ and $E$ determine the angle $\angle ABE$

Since $D$ lies on $EB$ and $A, B, C$ are collinear, $\angle DBC$ is the supplement of $\angle ABE$; then use triangle $BDC$

In triangle $ABE,$ $\angle ABE = 180°$ $- (60° + 40°)$ $= 80°.$

Since $A, B, C$ are collinear and $D$ lies on segment $EB,$ the angle $\angle DBC = 180° - 80° = 100°.$

In triangle $BDC,$ $\angle BDC = 180°$ $- (100° + 30°)$ $= 50°.$

Thus, the correct answer is B .