Calculate angle between lines projected on a plane

Hello All

This may be an easy answer, but I've been racking my brain about it for a couple hours and just cant figure it out. What I have is an inclined plane (30 degrees from horizontal). On this plane there are 2 lines, 20 degrees apart. What I need to figure out is how to calculate the angle between the two lines when they are projected to the horizontal plane, as shown in the picture (I am looking for the "In Plane Angle").

Regards,

JT

http://s271.photobucket.com/albums/j...t=Picture1.pnghttp://i271.photobucket.com/albums/j...g?t=1240969211

http://s271.photobucket.com/albums/j...t=Picture1.png