Because the angle sum of all quadrilaterals is 360 degrees.
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The sum of the angles in a trapezoid is 360 degrees, therefore, the forth angle in this problem is: 360 - (60+120+80) = 100 degrees
The 4 interior angles add up to 360 degrees
360 degrees.
the sum of the angles of a trapizoid equal 360 degrees.
The four interior angles of a trapezoid add up to 360 degrees