Exterior Angles Make 360°

Walk once round a polygon along its edge. At each corner you turn through the exterior angle. When you are back at the start, facing the way you began, you have turned exactly once all the way round.

So the exterior angles, one at each corner, add up to 360°, however many sides the polygon has. In the triangle, 110 + 120 + 130 = 360. With one exterior angle unknown, take the others from 360. The rule is Exterior angles of a polygon.

It needs a polygon with no corner pointing inwards, so the walk always turns the same way.