Question 747720
The sum of interior angles in a quadrilateral is (n-2)*180 where n is the number of sides.  (4-2)*180 = 360.<P>
In a regular quadrilateral all four interior angles are equal.  360/4 = 90.<P>
Supplementary angles have a sum of 180.  So the angles supplementary to each of these interior angles are 90 degrees.