How to create a circular diagram with Illustrator

in Illustrator CC-2014:

Create the circle:

Apply the stroke with the desired fill color:

Place guides in the center:

Copy circle and apply it to a new layer with Edit -> Paste in Place. Lock Layer 1 and select Layer 2:

Grab the Line Segment Tool and hold down the shift and drag the length in the center:

Use the Align panel and select the dragged line and circle:

Copy and paste in place a new line segment than rotate it with the rotate tool and it will tell you the percentage of rotation:

Select just the line segments and go to pathfinder and click outline then delete the bottom two anchor points with the direct selection tool:

Select everything on Layer 2 and click the outline again and delete all unneeded anchors with the Direct Selection Tool:

You should be left with 15% of the circle.