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To solve this problem we should remember that the sum of the adjacent angle of the parallelogram is equal to 180 degree.The adjacent angles of a parallelogram are supplementary (supplementary angles are two angles that add up to 180°)

Here the angle ADC and DCB are adjacent angle therefore their sum is equal to 180.

angle ADC+angle DCB=180

Given measure of angle DCB=60 degree

Substitute in the above equation we have

angle ADC+60=180

angle ADC=180-60

angle ADC=120 degree(answer)

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