All the angles of a $20-$sided polygon are the same. How much is each?

The sum of the angles of a polygon is $1980°$. What is the sum of the angles of a polygon with one side less? What about a polygon with one side more?

Two angles of a triangle are$40°$and $60°$.Calculate all its outer angles.

Compute all angles in the figure below.

Compute all outer angles of the quadrilateral shown below.

Compute all angles of the figure below:

Compute all angles of the given figure below:

Compute all angles of the given figure below:

Prove that in any triangle the outer angle at a vertex is equal to the sum of the inner angles at the other two vertices.