the interior angle is the supplement of he exterior angle
so interior+exterior=180 and
180-interior=exterior
the interior angle of a ngon is (n-2)/n times 180 degrees
decagon is 10 sided, 10gon
(10-2)/10 times 180=8/10 times 180=144 degrees
exterior=180-interior
exterior=180-144
exterior=36
interior angle is 144 degrees
exterior is 36 degrees