Please consider the complete question.
We can see from attached file that angle ABC is central of circle B that corresponds to arc AC. We can also see that angle ADC is inscribed angle of circle B and corresponds to arc AC.
We know measure of an inscribed angle is half the measure of central angle, when both subtends to same arc.
[tex]\angle ADC=\frac{1}{2}\cdot \angle ABC[/tex]
[tex]\angle ADC=\frac{1}{2}\cdot 70^{\circ}[/tex]
[tex]\angle ADC=35^{\circ}[/tex]
Therefore, the measure of angle ADC is 35 degrees.
