The size of interior angles of a polygon is given by:
[(n-2)180]/n
where n is the number of sides.
given that the exterior angle is 120°, the interior angle will be:
x+120=180
x=180-120
x=60°
thus plugging t=in the above formula and solving for n we get:
[(n-2)180]/n=60
(n-2)180=60n
180n-360=60n
180n-60n=360
120n=360
n=360/120
n=3
thus the polygon is a 3 sided polygon, otherwise known as triangle.