Answer:
60°
Step-by-step explanation:
The whole circunference is 360° from a central angle
so from an inscribed angle is 180°
notice that arc ABC + arc ADC make the whole circunference
so the inscribed angles that support those arc together must add to 180°
120°+b=180
b=60
from here we deduce that a cyclic quadrilateral (a quadrilateral that his points are on a circunference) oposite angles add up to 180
