Fold the triangle to get a right angle 90 degrees. Triangles are half of a rectangle or square. Quadrilateral shapes have 4 right angles which add up to 360 degrees. So half of that is 180 degrees.
Tear out the angles and placed them along a line. You will find that they will fit into a line and the line represents half a circle which is 180 degrees.
The video below explains about how parrallel and vertical angles have corresponding angles on opposite sides.