What is the measure of one angle of a regular 20-gon?

The measure of one interior angle of a regular polygon with 20 sides is 162°.

The general rule to find the measure of the interior angle of a polygon with n sides is:

( n - 2 ) * 180° / n

In the case of our 20-gon, n equals 20, so...

( 20 - 2 ) * 180° / 20

18 * 180° / 20

3240° / 20

162°