Not enough information. The total of the measures of the external angles of any polygon is 360 degrees, but without knowing that this is a regular polygon, just knowing that one of the exterior angles measures 20 degrees doesn't tell you anything about the measures of the rest of them. On the other hand if it is a regular polygon, then you know that all of the external angles are the same so you just divide 360 by 20.
John
My calculator said it, I believe it, that settles it