Question:
A regular polygon has sides of length 5 units and an exterior angle of 120 degrees. What is the perimeter of the polygon, in units?

Answer:
If an exterior angle has measure $120$ degrees, an interior angle has measure $60$ degrees.  A regular polygon with $60$ degree angles must be an equilateral triangle, so the perimeter is $3(5)=\boxed{15}$ units.