The sum of the interior angles of a quadrilateral are equal to 360^o. 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 360^o.

Example: How many degrees are in the fourth angle of a quadrilateral whose other three angles are 80^o and 110^o and 95^o? Answer: 360^o - 80^o - 110^o - 95^o = 75^o