Find the number of sides of a regular polygon if one interior angle is 120 degrees.

There are six sides

Because

First, the number of sides and the number of interior angles, both are the same

To find the sum of all internal angles, you use the formula:

(n-2) 180=z

where n is the number of sides or internal angles, and z is the angle sum. the letters can be anything

so now, if one interior angle is 120 degrees, then you get the total angle sum by multiplying 120 by n

so just write the equation, and replace z (angle sum) with 120n

(n-2) 180=120n

Solve it in any way you like

180n-120n=360

60n=360

n=6

there are six sides and internal angles! This is a hexagon.

FYI, a hexagon's internal angles add up to 120*6=720 degrees