Subtract the two known angles from 180 degrees to get the third unknown angle. For example, the triangle has one 30 degree angle and one 50 degree angle, so: 180 -- 30 -- 60 = 100. The missing angle is 100 degrees.
Check if the triangle has two equal sides (isosceles). The angles at the base of an isosceles triangle will be the same, so if you know one, you know the other. If you know the angle at the top of the triangle, subtract it from 180 then divide the answer by 2 to get the degrees of the two angles at the base of the triangle. For example, you know that the top angle is 48 degrees, so: 180 -- 48 = 132. Divide 132 by 2 to get 66, so the two base angles are 66 degrees.
Subtract an exterior angle from 180 if you know one interior and one exterior angle of the triangle. Because the angles of a straight line also add up to 180, you can find the missing interior angle of the triangle by subtracting the known exterior angle from 180. You then will know two angles on the interior of the triangle, and can find the third missing angle by subtracting the two known angles from 180.
Subtract three known angles from 360 to find the fourth missing angle in a quadrilateral. If you have one 60 degree angle, one 70 degree angle and one 30 degree angle, then 360 -- 60 -- 70 -- 30 = 200, and your missing angle is 200 degrees.
Subtract two known opposite angles from 360 if you have a quadrilateral with parallel opposite sides of equal length (parallelogram). Divide the remainder by two to find the remaining two missing angles, as opposite interior angles in a parallelogram are equal. For example, if your two opposite angles are 98, then 360 -- 98 -- 98 = 164. The two remaining opposite angles will be 164 divided by 2, or 82 degrees.
Transfer the angle measurements from known exterior angles in a quadrilateral to their opposite interior angles, as opposite interior/exterior angles are equal. If you know the exterior angles opposite all four interior angles, then you will know all of the interior angles. If you know adjacent exterior angles, then you can find the missing interior angles by subtracting the adjacent angle from 180 (angles in a straight line) to find the missing angle.