, 2012

Solution 1

The angle marked $\Box$ is vertically opposite the angle measuring $60\degree$. Since vertically opposite angles are equal, then the measure of the angle marked $\Box$ is also $60\degree$. Solution 2 The measure of a straight angle is $180\degree$. Together, the angle measuring $120\degree$ and the angle marked $\Box$ make up a straight angle. That is, $120\degree + \Box = 180\degree$. Therefore, the angle marked $\Box$ is $60\degree$.