When a transversal crosses three non-parallel lines, it forms eight angles. Out of these eight angles, four pairs of vertical angles are formed.
Vertical angles are formed when two lines intersect. They are opposite angles and are located across from each other. Vertical angles are equal in measure, which means that if one angle measures 'x' degrees, then the other angle will also measure 'x' degrees.
In the given scenario, when a transversal crosses three non-parallel lines, it creates eight angles. Out of these eight angles, four are pairs of vertical angles. This is because each intersection of two lines creates two pairs of vertical angles. Since there are three intersections of the lines, there are a total of six pairs of vertical angles. However, two of these pairs are formed by the same angles, which means that only four pairs of vertical angles are formed.
In conclusion, when a transversal crosses three non-parallel lines, it forms four pairs of vertical angles.