Question

How many sides does a regular polygon have if each exterior angle measures 20°?

Answer 1

Sum of exterior angles of any polygon is 360 degrees. 
Each exterior angle of a regular polygon = 360/n where n is the number of sides. 
So, n = 360/20 = 18. Your answer is D


Hope this helps.

Answer 2

The sum of the extrior angles always adds to 360
each exterior angle has to have a side to go with it

so
360/20=18 sides

answer is D