Answer:
ABC = 60
Step-by-step explanation:
The exterior angle is equal to the sum of the opposite interior angles
ACD = B+ A
100 = 40 + B
Subtract 40 from each side
100-40 = 40+B -40
60 = B
ABC = 60
Check the picture below.
notice that the triangle ADH, since the segment AL is an angle bisector, meaning it cuts the angle A in two equal halves, then the triangle ADH is only using half of A.