Answer: 120°
Step-by-step explanation:
Since it is a parallelogram, then opposite sides are parallel. Since ∠ADC is next to ∠DCB, they are supplementary angle (which means the sum of their angles is 180°).
∠ADC + ∠DCB = 180°
60 + ∠DCB = 180°
∠DCB = 120°
