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Parallelograms have opposite angles that are congruent and all 4 angles add to 360. Angle ABC is 80 and so is ADC because they are opposite. This adds up to 160. Angles DAB and DCB must add up to 200 because all the angles must sum to 360. Angles DAB and DCB are congruent because they are opposite so each is 100 to add to 200. Angle BCA is thus 60 because angles DCA (40) and BCA (60) must add to 100.

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