Vertical Angles

Vertical Angles are the angles opposite each other when two lines cross

They are called "Vertical" because they share the same Vertex (or corner point)

In this example, a° and b° are vertical angles.

The interesting thing here is that vertical angles are equal:

a° = b°

(in fact they are congruent angles)

Because b° is vertically opposite 40°, it must also be 40°

A full circle is 360°, so that leaves 360° - 2×40° = 280°

Angles a° and c° are also vertical angles (and must be equal), which means they are 140° each.

Answer: a = 140°, b = 40° and c = 140°.

Note: They are also called Vertically Opposite Angles, which is just a more exact way of saying the same thing.