The geometric method should be to make its three-dimensional model. I think this meeting will.

If algebra is used, it can be calculated by Euler formula:

Euler's geometric formula is: number of vertices+number of faces-number of edges =2.

It is known that a regular Pentagon has five vertices and five sides, and the number of faces is n.

Since the angle of the regular Pentagon is greater than 90 degrees, each vertex of the geometric body should be the common vertex of three faces, so the final number of vertices is 5*n/3.

One side of a solid must be the common side of two faces, so the total number of sides is 5*n/2.

Substitution means 5*n/3+n-5*n/2=2.

The solution is n= 12.

By the way, for geometry, you can look at the regular dodecahedron expansion in the encyclopedia.