Hans Walser, 
Find the indicated angle in Figure 1.
We will discuss a visual solution of this classical problem.
Fig. 1: Find the indicated angle
Of course we can draw the figure an will find 30ˇ. But how can we prove it?
By the way: we have an angle of 20ˇ in the figure. ItŐs not possible to construct an angle of 20ˇ with ruler and compass only.
The cyan triangle in fig. 2 is isosceles. In has the angles 80ˇ and 50ˇ. Hence the third angle is also 50ˇ.
Fig. 2: Isosceles triangle
First we introduce an equilateral triangle (red in figure 3).
Fig. 3: An equilateral triangle
Because of the cyan isosceles triangle in figure 2 we find now another isosceles triangle (yellow in figure 4) beneath the red equilateral triangle. It has angles of 20ˇ and twice 80ˇ.
Fig. 4: Another isosceles triangle
Finally we can fit a congruent yellow isosceles triangle on the other side of the red equilateral triangle (fig. 5).
Fig. 5: A second yellow triangle
The two yellow triangles in the position of figure 5 are part of a star (Fig. 6).
Fig. 6: The star
The star has a ninefold symmetry. In the center we have a regular nonagon.
Using two rhombuses (magenta in figure 7), a green equilateral triangle, and the symmetry of the star we find a rectangle (blue).
Fig. 7: A Rectangle
Hence the purple triangles in figure 8 are right triangles. Since one of the acute angles is 60ˇ, the other must be 30ˇ. This is the solution of our problem.
Fig. 8: Right triangles
We could solve our problem by help of a regular nonagon. This is due to the given angles. We cannot generalize the solution. Figure 9 gives a slightly modified problem that cannot be solved in this way.
Fig. 9: Modified problem
The angle is about 38.73789306893789ˇ.