A decagon is a polygon that has 10 sides and 10 interior angles ("deca" comes from the greek "deka" which means 10; the part "gon" is from the greek "gonia" for "corner or angle". So "decagon" means "10 corners" or "10 angles")
A regular decagon has each exterior angle equal to 36 degrees. To find this, you would divide 360 over 10. The formula you use is
E = 360/n
E = 360/10
E = 36
Where n is the number of sides and E is the exterior angle measure. Keep in mind this formula only works for regular polygons. The polygons must have the same side length all around, and the angles must be congruent as well.
Answer: 36