The size of interior angle=180-60=120°(angles at a point)
The equation to calculate the total angle measure of a polygon is ((number of sides)-2)×180°
Let x be the number of size
(x-2)×180°=120x
x-2=2x/3
3(x-2)=2x
3x-6=2x
x=6
So the polygon has 6 sides and each interior angle is 120°