In a parallelogram, opposite angles are equal, and adjacent angles are supplementary (they add up to $180^\circ$).

Solution:

Let one angle of the parallelogram be $60^\circ$.

• Since adjacent angles in a parallelogram are supplementary, the adjacent angle will be: $\text{Adjacent angle} = 180^\circ - 60^\circ = 120^\circ$

Thus, the measure of the adjacent angle is $120^\circ$.

Would you like further details or have any questions? Here are 5 related questions to consider:

1. What are the properties of a parallelogram related to its angles and sides?
2. How do you prove that opposite angles in a parallelogram are equal?
3. Can a parallelogram have all angles equal? What kind of parallelogram would that be?
4. If one angle of a parallelogram is $90^\circ$, what kind of parallelogram is it?
5. How would the solution change if the angle were $70^\circ$ instead of $60^\circ$?

Tip: In any quadrilateral, the sum of the interior angles is always $360^\circ$. This fact helps in solving for unknown angles in parallelograms and other quadrilaterals.