This has been an interesting exercise so far.
Saved projects.skp (121.4 KB)
This skp file shows my results for practice #8.
There are two problem areas, but darned if I know how I caused them nor do I know how to fix them.
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Good work Terry. I’m not certain why the reversed face on the one side but it can be reversed if you first turn on Hidden Geometry. Trace an edge of the circle on the bottom with the Line tool. The circle’s edge should revert from Profile (thick) to normal edges (thin). Then you can select and delete the face.
You have a hole which appears to have been caused by where you drew your edges.
I think you could avoid all of these isses along with the unneeded vertical edges at the bottom of the 90° with a change in your order of operations. I’m guessing you drew didn’t delete the face of the circle before extruding to the first height. I also expect you drew the edge for the vertical portion while the thing was still 2D. I think a better way to do this is delete the face in the circle before extruding and wait to outline the vertical part until after extruding to 10.
Edit to add a demo of a different approach to modeling this piece. Sorry about the sound. It was a long night. 
Saved projects.skp (92.4 KB)
Redrew it with much better results. I redrew it before seeing Dave’s video and I can appreciate Dave’s technique is faster and cleaner. It all adds to the learning experience.
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