Step-by-step explanation:
Angle formed at the circumference of the circle is half of the angle formed at the center of the same circle.
Hence,
[tex]m\angle ADC = \frac{1}{2} \times m\angle ABC \\ \\ \therefore \: m\angle ADC = \frac{1}{2} \times 40 \degree \\ \\ \therefore \:m\angle ADC = 20 \degree \\ \\ [/tex]
