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}