If you have two intersecting lines, there are going to be four angles that will all sum 360 degrees. The opposite angles will be identical. So, if you have one of the angles and it is 30 degrees, the opposite angle will be of 30 degrees as well. That adds 60 degrees, meaning that the remaining two angles will sum 300 degrees, and as they will also be identical, each will be of 150 degrees.
