OG详解-OG9 数学2 Q6

The correct answer is 70. Based on the figure, the angle with measure 110° and the angle vertical to the angle with measure x° are same side interior angles.
Since vertical angles are congruent, the angle vertical to the angle with measure x° also has measure x° . It’s given that lines s and t are parallel. Therefore, same side interior angles between lines s and t are supplementary. It follows that x+110 = 180. Subtracting 110 from both sides of this equation yields x = 70.