how many sides does a polygon have when the exterior angles are measured 20

Question

I need to work out how many sides a regular polygon has when it's exterior angles are 20 degrees.

Answer 1

For any regular polygon, the internal angle, A, of any n-sided polygon is given by A = 180 - 360/n.  Conversely, if we are given the internal angle, A, then the number of sides is gioven by  n = 360/(180-A).  If the exterior angle is 20° then the internal angle is 160°.  This means we have 18 sides in this polygon.