A quadrilateral is a polygon with four sides and four angles like a square, rectangle, diamond, or a trapezoid. A condition to be met for the angles of a quadrilateral is that the angles should sum up to 360 degrees. The four angles of a quadrilateral can be a combination of a right angle, acute angle, or an obtuse angle. Thus, for this problem, the missing angle is:
360°-100°-87°-106° = 67°
