Adjust the blue angles at A and B. Which combinations make it easy / difficult to solve for angle D?

This is a famous problem which looks simple but is in fact extremely difficult to solve with angle chasing. You may have a try, finding the unknown angle D in the figure. [br][br]The task here is not about directly solving this problem. You may consider varying the blue angles to other multiples of 10 degrees. [br][br]When would it be easy or difficult for finding the angle D? [br]When doing so, can you get a sense of what makes this problem difficult or easy to solve?[br][br]You may use the applet to quickly adjust the blue angles and see how the figure changes.[br]Do you have other suggestions of modifying this problem?[br][br]More info about this problem can be found in these slides: [br][url=https://dl.dropboxusercontent.com/u/4210789/me/mini/Langley/index.html]https://dl.dropboxusercontent.com/u/4210789/me/mini/Langley/index.html[/url]