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nati4966
Two Angles are Supplementary when they add up to 180 degrees. These two angles (140° and 40°) are Supplementary Angles, because they add up to 180°:
Notice that together they make a straight angle. But the angles don't have to be together. I hope it help you im not to good.
Supplementary means that both angles added up would equal 180 degrees: 

so 180- 120= 60 degrees.