Click on the links below to learn more about classifying quadrilaterals and calculating a missing fourth angle:

Link #1-Quad Quest #1 and #2 (interactive)

Link #2-Math.com (info and  QUIZ)

Link  #3-Regents Prep (interactive)

Link #4-MathsTeacher

Link #5-Cut the Knot (9nfo and quiz)

Link #6-Beacon Learning

Link #7-Kids News Room (quiz)

Calcuating a Missing Fourth Angle:

AAAMath