What is the interior angle of a twenty sided polygon?

A twenty-sided polygon is called an "icosagon" If it is a regular icosagon - all its sides and angles are equal - then it's interior angles are all 162 degrees. You can verify this by drawing a line to the centre from each angle.. There will be 20 small angles arranged in a circle around the centre. These must total 360 degrees. As 360 / 20 = 18, this leaves the other angles of each triangle to add up to 162, because the interior angles of a triangle total 180 degrees.