The sum of the interior angles of a quadrilateral are equal to 360o. To find the fourth angle of a quadrilateral when the other three angles are known, subtract the number of degrees in the other three angles from 360o.
Example: How many degrees are in the fourth angle of a quadrilateral whose other three angles are 80o and 110o and 95o? Answer: 360o - 80o - 110o - 95o = 75o
