To work out the interior angles, you need to know that angles on a straight line add up to 180°. In addition, you also need to know that angles around a point add up to 360°. When you need to find a missing angle, if the angle is on a line or in a triangle, take whatever value/values the angle/angles you have are and take it away from 180°. If the angle is around a point, (or in a square, where all angles are the same anyway) add however many values you have for the angles then take that away from 360°. Hope this helps! :)