Wow! There are a lot of ways to get to the same result in AutoCAD!
Thanks to all of you that responded to the AutoCAD Design Challenge. Feel free to continue responding (in the comments section of the original post) with your solutions! In the mean time I decided to start posting some of the existing solutions with graphics and demos to help you visualize. There’s something to be learned from each solution… even if a particular step isn't the quickest option for this particular drawing, it might be for a different drawing!
This solution combines several of your solutions into one. It assumes default (install) values, options, and settings and it primarily uses the command line interface (CLI). I counted every click and keystroke (that was the REAL challenge) and included them in parenthesis at the end of each step. If you tend to use buttons and menus instead of the CLI, don’t let this scare you. The buttons and menus are a more intuitive and graphical way to access commands but when it comes to clicks and picks, the CLI is usually more efficient.
You can view a video of this solution or follow the steps below.
X <enter> pick <enter> (Explode the polyline - 4)
O <enter> E <enter> <enter> 4 <enter> pick pick pick pick <enter> (Offset the two top lines 4 units up, automatically erasing the two original lines - 12)
F <enter> M <enter> pick pick pick pick pick pick <enter> (Fillet the new lines and the two sets of parallel lines. The multiple options saves a few clicks. Also, notice fillet of two parallel lines automatically creates a 180deg arc between them - 11)
pick pick pick <esc> (Grip edit to move the center of the circle to the center of the arc - 3)
pick pick pick <esc> (Gotta love grips! - 3)
pick pick erase (In this case it saves you a click to select the Erase tool from the toolbar/ribbon rather than “3 <enter>”... assuming it’s currently displayed, which it is by default - 3)
So, if I counted correctly, the grand total for this solution is 36 picks, clicks, and keystrokes. Not bad!