The arcs AC, AB, and BC must add up to 360º. Then:
arc AC + 120º + 100º = 360º
arc AC = 360º - 120º - 100º = 140º
Now, the inscribed angle ABC is one-half the value of the arc AC. Therefore:
[tex]m\angle\text{ABC}=\frac{140^{}^{\circ}}{2}=70^{\circ}[/tex](option B)
